My 2 year old gets a bit funny when the usual educators aren't there, she likes to see a familiar face at drop off. Agree with talking to them to find out if there's been any staff changes/holidays in the room lately.
Also when DD was really bad at drop offs I'd try to call about half an hour later to see if she'd settled down. Every time they said she was fine 5 min after I'd left.
We had a few weeks of tears at drop off when she transitioned from the baby room to the toddler room also, if your DS has recently moved rooms it could be the cause? @Unschooling4 your comments are not helpful, 2 year olds cry hysterically for lots of reasons, doesn't mean their being abused!