Since you noted your son is a pitcher, I thought I would add there is a lot of variation of pitchers on most rosters as you might expect. A lot of it depends on what happens during the actual college baseball season. Some pitchers are on innings limits going into the Summer, so some will depart the team after they reach those. The teams are always adding pitchers as the Summer goes on. Other kids get hurt, as pitchers are particularly vulnerable if they are used multiple times per week versus a game or two in the college season. And some kids have not built up much endurance--as one poster noted, there are a lot of kids who Redshirted or pitched very little (as my son did for his college team) and are now expected to contribute. My son went from being a typical 1-2 inning pitcher to reaching 90 pitches in his final outing after six weeks building up his arm.
Your son should know that the experience will be a real plus for him as both a baseball player and as a life experience living away from home with a host family. But he needs to be fully committed and see it through to the end. Lots of kids do not seem to understand what it means to sign a contract and stay committed. There were two kids from the same college team who quit my son's team after two weeks, and they will be blackballed from the league and their coach may have a tough time placing other kids in the league as a consequence of their action.
It is an absolute blast to go to the games and experience it all. With your son a pitcher, try to plan a whole week to make sure you get a chance to see him in action once or twice.
P.S. Regarding fees, you will likely be expected to make a uniform deposit at the beginning of the Summer, but otherwise do not have to pay anything. We did replenish my son's ATM account a lot as the food they got was somewhat limited and they are young athletes who like to eat a lot.