Is your grandma not able to let him out or is she choosing not to or forgetting? I would make the rules clear with her, if she wants to let him out of his room where he was trained, then she needs to let him out consistently. You could even set a timer to remind her, or hire a dogwalker. Since the pee boxes are working, having them in more visible areas could help, and confining him to one or two rooms would also be beneficial.
Good luck
