Although sometimes it doesn't seem like it, this is the HIGH SCHOOL baseball web. The focus does not have to be playing on a top national team in order to play in college, it can be playing in high school, if that is fun, and if you're at a school that allows kids to be on the team who "only" find it fun. Not all schools expect full-out effort from everyone. Was your son frustrated in HS because he wasn't playing much? or because the team wasn't good? or didn't like the other kids? or the coach? or because he didn't meet his own expectations?
My oldest son played high school JV golf for 4 years. There were 5 kids in his grade who were superstuds and they were the varsity. My son was naturally good enough to stay in the next group. If he had worked hard at it, maybe he would have cracked that group, but he didn't seem to care that much (or maybe he didn't think it would help), and so he did not do the extra work, the lessons, the weightlifting, the summer tournaments, etc. To be honest, I never quite understood why he kept with it for 4 years, but he said he liked the practices. Now he plays golf for fun.
Note: he dropped baseball after 8th grade, but umpired rec-league baseball all through high school, earned quite a bit of money.