When my DD was like this I moved her to another nursery. She was crying every morning when she'd previously loved it. Only one nursery nurse could cheer her up and she did not work full time in DD's area. I moved DD and she adored her new nursery and I never had one tear or reluctance to go.
I know a lot of people say 'oh they're fine when you leave' or 'they're just pulling on your heart strings' but I think this devalues the child's opinion. Or your DS may prefer a more homely childminder environment or mix of CM and 15 hours pre-school.