@ 13 size doesn't matter as much as experience. At that age some kids will have 8 years of coaching already. You may want to find a team in a league that has, "A" and "B" (or Jr & SR) teams. These leagues load the "A" or Sr teams with most of the experienced players, while the "B" or Jr teams teach similar experience levels, at a more relaxed pace.

MY NAME IS COACH CLEO OF THE WOODCREST GENERALS YOUTHFOOTBALL/CHEER (323)757-9714 GIVE ME A CALL

He should have been playing already, experience is everything!

Investigate camp opportunities where he can be introduced to proper instruction, form and techniques. His abilities can be evaluated, you will received input and answers to whatever questions you and he may have. Good luck.