I think the decline in Jack is related to an upswing in Jackson, Jaxon, Jax, etc.  I think it's great that you are using John as the formal name and if you are going for distinctive (or if he chooses to) than he can be called John. 

I agree that Jack as a nickname for John is very rare these days.  I also agree that Jackson/Jaxon seems to be replacing Jack, I know several Jacksons age 10 and under.  However, none of the Jacksons I know ever shorten to Jack, they all go by their full names.  I would love to see a "real" Jack in amongst all the versions of Jackson.

We have a 9 year old who is a, "Just Jack."

One year, he had another Jack in his class at the Y, and one year he had a Jackson in preschool. Other than that, we haven't met duplicates, but people frequently will say, "Oh, we have a cousin/friend/neighbor named Jack."

I think it probably has peaked, based mainly on what I hear at the playground and from my sister, who lives in the land of trendy namers. Most of the Jacks she knows are around kindergarten/first grade age.

As others have noted, there are now Jax and Jaxons, and of course, for most of his life he won't be only with boys of his same age.

Fwiw, we get lots of compliments on our son's name, and if you're using John you'll always have options.

Like previous posters, I think it might have peaked. Most of the Jack's seem school age now. Between that and Jackson/Jaxon becoming more popular a John nn Jack will probably stand out more than he would 5 years ago.

I actually know 3 Jacks under the age of 3. All there first names are Jack. One lives in Louisiana, one Tennessee, and one in Texas. I also know a few young Jacksons. But I love the name Jack. And I like that your planning on using John. I say if you love it, go for it :)

My daughter has a couple Jacks in her grade level at school (1st grade), but I don't know that many Jacks under three. Only one, in fact...a new baby. I think Jack is still pretty popular for families who like the Max/Sam/Gus/Hank family of names but it's not as fresh-sounding to first-time parents.  So I would guess that most little Jacks are younger siblings and the family naming style was set a few years ago...just a theory, though!  It's a great, classic name and sounds good on little boys and old men so it's probably going to stay popular for a while. :)

my daughter is 4 and has one Jack in her class. that is the only one we know, but boy are there lots of jacksons popping up.

For awhile, there seemed to be a Jack or two in every class my 2008 and 2010 babies were in. There are none in their new classes this year, but I have noticed a few in the surrounding classes.  It is definitely one of the most consistent/popular names for boys in my circles.  We don't have any Jacksons/Jaxons/etc. (that I know of) in these parts, though.  We are definitely in a yupster circle.  Jack does not have quite the saturation of Olivia and Isabell(a) and Sophia, but it is not far off.