The behaviours your grandson are exhibiting are those of anxiety but I don't see any evidence from your posting that this child is suffering from an anxiety disorder. The best thing one can do for non-severe anxiety is simply to "lessen the anxiety". Can you lessen the times he speaks to his mother or at least, shorten the minutes of his telephone conversation? Can you not speak of her in his presence (unless mandatory)? Can you pretend (at least for a short while) that she doesn't exist?
I have seen this type of behaviour before whereby a child will "choose" not to be involved with a mentally ill person (even if a parent). I think I would call this behaviour "trying to survive". Will your grandson be staying with you all summer - if so, try to plan relaxing and fun things for him to do. Try to "lessen the anxiety" by focusing on sports, or video games, or trips to the park, or playdates with friends, trips of the mall, camp (if you can afford it), swimming at the local pool, etc. If there are still issues in the fall, then I might suggest you speak to the psychologist at the school or contact your own family physician - either one should be able to help you I wish you the best ....
What a lucky boy to have a caring Gran like you,Sounds like he could do with a bit of new Therapy ,always good to get him a Pediatrician and they will refer you to someone or ask the School if they have a school councillor he can talk to.Hes been through a lot my guess is and it has given him some Anxiety ,maybe at the thought he may have to go back to live the old Life, sad isnt it ,but he has you and you have him, are there any interests he has you can encourage,are there some Men in his life ie Granpa or an Uncle or Friend can do some guy thing with him?Find something hes good at and likes to do and build up his self Esteem. Good Luck