classes in your "off" semester, if you want (basically, go year-round) (which is a fairly new thing; they used to lack capacity to allow students to do that).
Your grades don't have to be stellar to get in. They do have to be stellar to get (and keep) an academic scholarship. Also, they're really student-oriented, which can be helpful for any student just starting college.
All honorably returned missionaries get a $1000 scholarship — all your son has to do is ask (or fill out the correct form).