Do you hit fungos to him?
That he has below average baserunning skills might point to a lack of baseball instincts. Measurables are more important but if you aren't terrorising the ball baseball instincts do help getting into the lineup. When he makes bad baserunning decisions coaches probably think that will transfer to other areas of the game too so they don't trust him to play tougher positions than 1b
Work on getting stronger and bigger and improving your hitting and running but also work on those baseball instincts.
I had that issue in my first years of playing too, hit the ball hard but was sloppy on field and bases so I didn't play so much. Tightened that up and then I played most of the time.
Not everyone can be a stud on the bases but you can for sure make sure that you don't make mental mistakes.