My Dd who was just three went from the perfect angel when getting dropped at day care to exactly what your son is doing. The careers said there was no other issues and generally through the day she was fined.

Once I had DS she was fine. I think they worry about their Mums and us being ok. Children have such a intuition that us adults don't seem to understand at times, they pick up on the tiniest things that we may not even realize are bothering us.

Maybe ask a carer when your mum arrives to come up and talk to your son and distract him with something to do to try and reduce the time which he has to think about being dropped off.