Developing arm strength

That sounds almost impossible - just maturing and getting older naturally should have led to an increase in his velocity.

Could you give us more information about him

His size as a freshman to senior year
How much he lifted from FR to SR year
How much did he stretch from same years
How much long tossing
Are his college coaches having him do things differently than when he was in HS

There might be something in that information to help us be able to give you a better solution.

Throwing 80 as a HS freshman doesn't sound like a lack of arm strength. I agree with coach2709, it seems more likely the coach(es) messed up something else in terms of mechanics/timing (assuming there's no health issue involved).

Since he is now pitching in college why not leave that to his college coaching staff?