Could he afford to buy the flat from you with the proviso you can stay there when required?
I know the OP has given us an update, but felt I had to comment on this suggestion.
Picture the scene a couple of years down the line after the above scenario has happened. GS now has a live-in girlfriend maybe.
“What do you mean, your grandparents want to come and stay here?For how long? Oh, for goodness sake! Where are we all going to sleep? Well, I’m not sleeping on an air bed! Tell them to stay in a hotel, they can afford it! This is our flat now, remember, not theirs!”
You’ll think it could never happen…..