I was worried about this with my boy too. When my mum babysat him, he just played on the floor and at some point she said "want a cuddle?" and he sat up with her and went to sleep. I was thinking there would be tears and screaming for booby and he'd be very upset.... but he was fine! I've since left him with his dad (we're divorced) and the same thing happened, no crying and no upset. My boy was a bit older than yours - but your mum sounds like a lovely grandma so I'm sure he'll be ok with her