Bowie is ok, but Beaumont sounds very harsh to me.  I also think of Beaumont, Texas-which makes Beaumont seem more like a place name to me and less like a surname name.  I think something like Beauregard sounds more dignified and could still give you the nickname Bowie.  There is also the surname Bowman.

Of your runners up, Truman/Tru would be my favorite.  I think Brock works better as a given name instead of a nickname for Broderick.  I've heard of more girls named Harley than boys, so if the name "going girl" is a concern for you, I'd suggest avoiding Harlan/Harley.  

Some other surname names with nicknames you might like:

Lincoln/Linc

Coleman/Cole

Harmon/Harm

Tobin/Toby

Fletcher/Fletch

Everett/Rhet

Beaumont is a little fancy for my taste, but there definitely isn't anything wrong with it! I think the nn Bowie is adorable for a little boy, although if I saw it on its own I would pronounce it like Boo-wie instead of Bow-ie because that is how I see it pronounced most often in my area. Just wanted to let you know that you might come across that if you go with Beaumont nn Bowie. I don't think you would see this problem if you introduced him as Beaumont first though!

Personally, I think Truman nn Tru is more easily wearable and would be easier to find sibling names to match if that is a concern later on. Broderick carries a strong association to Matthew Broderick for me, this might not be the same for others. It seems like it is midway between the styles of Beaumont and Truman. Harlan nn Harley is a little gender ambiguous to me, as I have only seen the name Harley on girls. Harlan itself seems less familiar than the others, but is a nice, unusual alternative to all the trendy boys names with the -n ending.

Truman is my personal favorite, but you have a lot of good options!

Beaumont seems a bit hefty for a first name.  I think if I wanted the nn Bowie I'd make the full name Bowen or Beau.  Still, I prefer Beaumont to Harlan/Harley, which is my least favorite of your list.  I just can't get my mind to see Harley and think "classy."  Your other names I could see on super-classy guys, but not Harley.

I'm going to disagree with most of the previous posters. While Beaumont is not a name I'd use I do like it. I like the option of Bowie as a nickname although I'd personally pick Beau, but either would work. I don't think it's too much, esp if you use one of the nicknames.

Truman is my least favourite of your list.

Harlan/Harley read totally male to me and I don't mind it. I also don't mind Broderick/Brock. I guess it would depend on your surname as to which is my favourite but other than Truman I like all of them.

I agree that Beaumont is a bit fancy for my tastes, but fine, and particularly cool if that's a family name or otherwise has some personal meaning.  Bowie seems like a great nn that he could use and fit in if he felt that Beaumont were a bit much.  My favorite from your list is actually Truman/Tru, followed by Harlan without the nn Harley (Harlan is an old school southern name to me, and Harley does not sound particularly great to me--I feel like a boy named Harley might have a sister named Crystal or Brandy).  Broderick is ok, I guess, although this is a family name for me and I don't think of it as a first name--however, one of my Broderick cousins goes by Brody, which might interest you.  Brock sounds very preppy (or faux-preppy, in the sense that I think some traditionally preppy names have been adopted by the new cowboys), which is fine, but it's not my style.  

How do you pronounce Beaumont and Bowie? Neither one has an automatically-intuitive pronunciation for me, and Beaumont has the added problem of too many letters in that first syllable, making it hard to spell.

I dislike surname-names generally, although if there's a strong family tradition attached, I can forgive them on a case-by-case basis.

I typically dislike place names, but I actually like Beaumont.  I don't know why, but I do.  Some things to think about with the nickname though:  Zoe and Chloe are both wildly popular right now, and it might be rough for little Bowie to be confused with little Zoe.  Also, there is only one association that Bowie can ever carry for me, and that is David Bowie.  I would be cool with that, but it's certainly something to think about.  

I like Truman.  I've long wondered that it hasn't taken its place in popularity along with the other presidential surnames.  

Also, I think it's pretty typical for parents to second-guess their names when it's about time to actually have the baby.  Naming a child is a huge responsibility, and you're probably just getting cold feet because it's a big deal, and you want to get it right.  

I grew up in Beaumont, Texas so Beaumont seems like a really strange choice for a first name to me.  I love Beau and Bowie and also Truman/Tru and Broderick/Brock.  Another unexpected nickname I like is Rhett from Everett, Garrett, or Barrett. And a friend recently named her son Campbell and calls him Camp.

I also prefer Brody over Brock as a nn for Broderick. (and yes, I do think of the actor.). The only Brocks I know are a little older than me, so it's similar to Jared and Mindy to me.

The Harley for little girls is a spillover from Marley, I think.

I really like Bowie, and although Beaumont seems like a little much, it could work.  I don't see the other way to pronounce that other posters have mentioned, and thought it was very easy to say when reading it.

Boh-ee  boh-mont ?

I much prefer Harlan to Harley. Harley has the strong association with the motorcycles, plus I think of it more as a female name (because I've met two Harleys-both female).

Of the other three, I prefer Broderick. And I like Bowie as the full name rather than Beaumont. I don't think pronunciation is an issue with Beaumont though.

I hear Truman and immediately think of Harry Truman and WWII.

I like Beaumont, love Bowie.  Just by looking at it, I would pronounce it like David Bowie's name.  I'm not that into the runners up.  Maybe Broderick.

I'm not a fan of surnames as first names, but I guess Truman is the best of the bunch.  Beaumont sounds pretentious, and I'd laugh out loud if I overheard someone call their child Bowie.  Broderick = Matthew, and I'd be concerned that he'd get involved in some scandal.  Harlan sounds hickish, and Harley as a given name is used more frequently on girls than boys.

i may be in the minority, but i like Beaumont, and the prn. doesn;t pose a problem for me. you could use Beau or Monty as a nn, should you choose.

I've grown up around Beaumont Tx all my life and while it is not my style I think it makes a nice boy name. It fits with the place name and surname trends that are popular now and is a little unexpected.  The nickname Bowie immediately brings to mind the knife for me which is a very masculine connection. I also know many grown who who go by Boo and Bo or Beaux the latter has a heart breaker feel to me. If it has been your top choice all along and you are prepared to spell the name and are ok with all the nickname possibilities (kids often create their own)- I would stick with Beaumont. 

I like Broderick and Truman as well but Harlan makes me think of a girl Harley is common for pets as well. Howevee it is a lovely name too. 

Bowman would be easier to spell and would eliminate the Monty nn. I don't think Beaumont is going to be too hard for people though. Just unexpected.Which is nice imho.