My daughter came to live with me when she was 7. Her birth mom lied to her during their one hour visits. She would get very upset. Do you know what his mom is telling him?
I suggest you stop trying to convince him you're alive. Talk about his life with you and his life with Mom. Consider that he feels like you died. Sounds like there wasn't a few weeks of him being with his Mom and with you. As far as he's concerned you left him which feels like death.
Why could he not move with you? Is the state Children's Division involved? If so, I suggest calling them for help. Was there a couple of weeks during which he saw both of you? Do you get along with his mom? Is he torn about loyalty between the two of you.
Was he with you because his mom couldn't keep him safe? Is she able to keep him safe now? Has she had help in knowing about his needs and how to provide them? Your home and the way you cared for him is, no doubt, different than the way his mom cares for him. That is why you need to talk with him about life with you and life with Mom. It would be best if you can honestly tell him you both love him. It's best if you and his mom are friendly with each other. Find a way to help him adjust to his new mom and new home.
Sympathize with him about how hard it is for him. Show him that you're still alive by continuing to talk with him. Don't talk about death. Talk about things that are happening now and relate them to what happened when he lived with you.
My grandson has learning difficulties. He's in a special ed classroom. Once he gets something in his head, he holds onto it even when we try to tell him something different. I've learned to just let what he believes go. I don't try to convince him differently.
Is your grandson getting help? It's essential that he and his mom have.professional help to deal with his developemental delays before he starts school.