R.K.
When he starts to hit or push other kids, intervene right away. Pick him up and take him to a bench and make him sit on the bench for about 5 minutes. As you pick him up say, 'we don't hit or push other kids', or somthing similar. After he sits quietly for about 5 minutes ask him if he can play nice, if he says yes let him go play again. If he starts hitting or pushing again, pick him up and sit him on the bench with you again.
By stopping the bad behavior right away you send him a clear message his behaior is not acceptable. By repeating the intervention he sees it will not be tolerated even for a minute.
Since he behaves at daycare but has issues at the park, watch carefully exactly what is going on. He may be only pushing or hitting back. it is often the second child who gets caught. If another child is hitting or pushing first, tell his/her mom to monitor her child.