As we talk about often around here, has a lot to do with his specific situation which includes performance, feedback from coach, etc. As you can see from the different perspectives provided in the the responses already, it also has a lot to do with how the coaching staff approaches each fall scrimmage.
In my opinion, the best advice you can give him is to self-advocate. The first thing he should do is to find out if his HC and/or PC hold exit meetings following fall ball. If not, recommend that he talk with his PC to find out where he fits in once fall ball has finished up.
As has already been shared, most freshmen get limited innings in the spring unless they dominate in fall ball. I think every freshmen who shows up in the fall wants and sometimes expects to make a significant contribution once the spring season starts. Makes sense since there most recent playing experience usually includes significant playing time. Just remind him, if someone else doesn't first, that he needs to have realistic expectations. Did he get significant innings on varsity HS team as a freshman? Probably not but doubt he was looking at transferring.
As for advice, I respect CT's thoughts and believe that we need to give our kids space once they move on to college. However, I think it is important for your son to know that you are available should he need guidance. Hell, I still ask my Dad for advice occasionally and respect his opinion even if we don't always agree.