Well, I think if that's the ONLY time you're going to visit the campus, you shouldn't do it when school is out. But if you can go back to his top picks when they're in session, then I don't think going during the summer wouldn't be a problem.
I agree that you absolutely need to see a campus in action, but a preliminary visit could be handled in the summer. Especially on a big campus, a summer visit can let your son (and you!) get the lay of the land when things are less crazed. It might allow him to shave a few days off a visit during the school year so he can just go for one day and night, rather than needing a longer visit to get everything done. Just call ahead, obviously, to be sure professors are around and that sort of thing.
If you to don't mind going back the schools later, then I'd say go for it! Do the visits now.
