V.G.
It may be one of those things that he'll eventually get used to. Maybe see if Grandma would be willing to come over for dinner, at the apartment. Explain to her ahead of time the problems you are running into. Then when she is there, she can "talk up" the apartment - how nice his room is, little things like that. If she comes over on a regular basis, he may begin to realize that even though he's living away from Grandma, that she is still around and there for him. It's not leaving Grandma, just living somewhere else.