Building on what RJM said... 1) Support the team. With 32-40 guys on the team there will be a lot of kids that never see the field. If your son is one of those, be there for the team and support all the kids who are playing. 2) Don't complain about playing time to other parents or why little Johnny should be out there instead of some other kid. 3) Don't be "that parent" that talks about how Johnny was all league, all state etc as most everyone on the team was 4) tell your son how happy you are for him to be playing college baseball, that you love him and love coming to see him and the team and that you get to still watch baseball and 5) Don't give advice about what he needs to be doing or not doing... other than keeping grades up!
If he is fortunate enough to play, be humble and happy for him. No one likes parents who only cheer or talk about their kids. If someone says he had a great game or play, just say thank you and glad that he was able to help the team...