Even if you have visited schools in the summer, the in-semester visit is important, as other have said, he may not like the school when he stays with players. Also, sure he'll probably get an offer when on a visit, but for D3 schools there are various kinds of "offer" - they could say "we don't have a slot for you, but we hope you'll get in and you'll have a spot on the team," which you might like less than a more definite "we will get you through admissions" at another school. You have to be in there with the coach to ask exactly what kind of offer it is, unless your son has already asked on the phone. If the top choice coach has said on the phone "I have a list of xx players who are guaranteed admission and you are on it" then I think you could visit only there.