M.P.
Ask Grama if you can move everything up above your child's reach or move it to another room to which you'll keep the door closed. I've done just that for other visitors to my home. Most everythingI have now at a child's level is not fragile and I dont mind if the child plays with it. Could that be the case with Grama?
At the very least I'd tell Grama your concern.
Another idea is to take a play pen and put him there with his toys from time to time. You might be surprised at how quickly he learns to not touch. It will take a few hours of follow up but he just might be able to behave in a manner acceptable to Grama.
Good luck! M.