T.H.
Ugh, that sucks. My SIL has a son who ran into conflicts with boys in his kindergarten class encouraging him to misbehave. He's really a great kid, loves to help the teacher, excells in his work. One day my SIL brought him into the classroom and he started to do something for the teacher. One of the other boys came up and said "don't help the teacher, that's lame" and my SIL heard him. She went over to her son, and told him that boy doesn't know what he's talking about, and that helping the teacher is a very good thing to do. She's had talks with him several times about that boy not being a real friend and how she doesn't like his behaviour. She doesn't 'attack' the boy, just tells him how his behavoir is poor and could be better. She encourages him to be himself and find other friends.
Your poor son is caught in a hard place. He's getting pressure from his 'friends' and his parents are punishing him. Have you given him options or tools to deal with these kids? He's struggling with bullying behaviour at a very young age, and it's so confusing and he probably has NO idea how to handle it all. Instead of telling him what not to do, maybe role play some scenarios and teach him what to do and what to say to these bratty boys. This is a perfect teaching and bonding opportunity for you.
Oh and yeah I agree with the other posts, it sounds like this daycare isn't a great fit for you. Bullying should not be permitted, especially in a group so young.