If all else fails (I'm with McRabbet on this one), change the locks while he's out and leave his stuff in the carport or under a tarp. Depends on how tough a nut you have to crack.
I was lucky, my 24-YO and his long-time GF came down after we did (after graduating), at our invitation. They stayed with us while finding jobs, then actively looked for and scored a nice apt in Durham. The rules were that it was to be a temporary arrangement (about 6 months) and I think it came out to about 8 for everything. Yep, they pay their own way now. I'm not so sure when my 21-YO finishes if we're gonna be that lucky again 8-O