Measure the distances between tire and sides of fork, a well as top, also at the rear with chain stays.
as noted, if too close it's not worth the risk, but if ok a 28mm will be a bit more comfortable--at the right pressure!
I weigh 135 and can run 70, 75psi on my 28mm bike front tire.
buuuut, one factor to take into account, the bike may fit now, but by next summer he may have grown 2 feet taller, so beware of that.
Last edited by djb; 09-04-21 at 06:24 AM.