He should be fine. In seventh grade, I pitched in the full 60 ft six inches as age 12. I was probably your sons height, maybe an inch or two shorter and weighed like 110. So I wasn't big, either. To top this, my mechanics were horrendous. I stepped openly, turned my body, then just side armed the ball... Lol. I threw quite a few pitches like that and turned out fine, so I'd say let him go for it!