Do you work full time? I wonder if it would help to do a few visits that aren't dropping him off e.g. go in to talk to the teachers or pay a bill, or he have a little play then you head home together. He has obviously worked himself up about it and it might help for him to develop some positive associations that aren't about you leaving him there and having to go to work. It is so hard when they are crying as you leave. I wouldn't be able to handle it either.