I would have your son do some research on the LHPs who are currently getting innings at the DIIIs he's interested in. Some will have Perfect Game profiles with their high school ht/wt and velocity numbers. He might even find some guys who have numbers similar to his. Of course, he'll have to dig a little deeper to find out how those guys have progressed since high school, but it will give him some context.
His expectations of playing time are also a factor. His academics might help him make a team where he won't play much. If he isn't interested in that kind of experience, he needs to be clear about that.