52-14 = 100 inches
55-15 = 99 inches
56-15 = 101 inches
For all intents and purposes, those three gear choices are pretty much the same.
Based on what you've told us and the pics of your son on FB, I would guess the 58 Fuji would fit better than the 56. I'm shorter than your son, and my fitter, who is a very accomplished track fitter, would put me on a 61 Fuji for sprinting. You might try to find a track fitter online who can provide some guidance via online videos if you do not have someone local.