My son is 3'5" and he is super slow and 30 pounds overweight and can't catch or throw, but he could run circles around your son!
When he is that short, you really should just use inches. For example, my son is 41". It's kinda like when those annoying moms say their kid is a baby for too long....."He's growing up so fast; next week he'll be 132 months old"
