Has he started shaving yet? That is about the time to get on a strength and weight training program. This is usually done outside of baseball, with a proper sports specific trainer. There are weight gainers (supplements) out there, too. It was winter of freshman year when my son started on a weight and strength program. There are running coaches out there, too, and maybe just a few lessons will help. Sub 7 is usually the metric coaches look for. Unless your son consistently hit home runs.
It can't be stressed enough: you gotta be able to hit.