Tell your son to be himself. If he likes wearing athletic shorts and a tee shirt, wear it. If he likes wearing khacki pants and a button shirt, wear it. Coaches are looking at his baseball talent, how he will fit in on the team (specifically whether he is a team player), and whether he can make it academically. They aren't going to evaluate him on what he is wearing. He wants to project who he really is, not who he thinks they want to see. At some point people always revert back to who they really are.
This is possibly the worst advice ever posted on the HSBBWEB.
Agreed Redbird.
My son's travel team enforces khaki (or similar) shorts/pants and a collared shirt when they are on team travel. That means flights, meals, everything they do in public while travelling with the team. If they are on the field for practices or other things where they are not in uniform, they must be in team practice gear only.