Me personally i would work on muscle mass and quickness in both the feet and hands. Most of the time the guard is going to be pulling to the left or right. I would also recommend a football camp, they happen all the time in summer. Great way for him to find what is "best" position on the field would be.
I would work on cutting out all bad sugars(pop, candy, chips) if you haven't already.
Thats all i ever did, didn't take anything to beef up, just lifted weights worked on speed everyday thru out high school. And i made all state center 2 years in a row for lowly o' SD.
