I just put my daughter into daycare also! I was soooo hard, but I am going to school now and she is at the daycare there. She actually did very well! She is 2, so a lil older than your boy, but I made a big deal out of calling it school instead of daycare. If you and your son know any older children explain to him that they go to school, and he is going to school just like them. I tried to be as calm and coolabout the whole thing and Especially around drop off time (even though I was a bundle of emotion inside I didn't let her see it). I always talk about something fun that she can do while she is there just before I drop her off, and when I do drop her off I just say i will be back in a little bit, kiss her hug her and then go, and don't look back. If you look back they think you may change your mind about leaving them there.
Another possible help would be, get together with one of the other moms that have their child there and have a play date outside of daycare with that child. then when he goes to school you can tell him he will see his friend there. Or just tell him he is going to play.
These are some of the things I did and so far they seemed to work. Althought the 2nd week the whole way up to school she was saying No School, but she got over that lil stage. Sometimes I think it is worse for me then it really is for her. As moms we alwyas want to be wiht our children and protect them from anything they don't like or enjoy.
Another thing is, when my daughter was 13 months old she was going through another attachment phase, maybe that amplifies the situation. Eventually your son will love being there with the other kids so don't worry , it just may take some time. BUT HANG IN THERE! I know how hard it is!!!!!! I cried when I dropped her off the first day! I was a wreck... But guess what .. They will be ok.
Good luck I hope some of the advice helps, it is a tough change to go through for both you and your son.