does he know the number of sides of shapes? it is difficulat to figure out what is the problem, maybe he doesn't notice relation between lines in an object, like crossing lines, or parallel ones. so you can try with that. my son had a prblem of corners in shapes so he could draw it when he would position dots at corners first.
also if he likes trapnsportation vehicles and signs you can try to introduce letters and shapes that way, like stop sign.