Conflicts between pre-schoolers are quite normal, usually start from fighting for a toy or a company. You could verbally tell your son that pushing or shoving is rude. Never use the punishment again, as punishment or threaten will never work things out but cause negative effect on the child's self-esteem. Let the teachers know that it is their responsibility to deal with the conflicts between your son and other kids that happened in the daycare, and they should know how to help your child to develop his social skills instead of complaining to the parents.