Cav, your son is only 13. Who's to say he won't eventually be a hard thrower? Or will? My son maybe threw 65 at 13. 70 at 14. 75 at 15. 80 at 16. 89 at 17. Now can touch 92 and is in the minors. BTW he's 5'10" in his shoes.
I would suggest the path Bum, Jr. followed:
1) Consistent long-toss year round
2) Good pitching instructor
And when he's about 15:
3) Plyometrics
4) Band work
5) Weights in offseason (light in season)
It's far too early to fear your son is lacking velocity. In fact, it's probably good, so he isn't used too much and burns out his arm too early.
Good luck.