MD,
It happens at all levels. My older son was a part of a group of 14 signees at a D-II in South Carolina 4 years ago; 3 remain there today. What you'd like to know from the coach is how many folks are being actively recruited at your son's position, but there are truly no guarantees on playing time anywhere.
At D III, with no schollys, players seem to drop off at odder times (one did this semester for example at my sons' school after he signed up for spring classes, when baseball players get early access to classes)than we saw at D-II, but as players get older, if playing time is not to their liking, they tend to transfer, or drop off. Baseball takes a lot of time at all levels, and many players do a mental cost-benefit analysis along the way.....