Bullying, need help

My son is 3 and there is this 5 year old boy that bullies him. He pushes him for no reason he says really mean things to him, for no reason. My son honestly does not even talk to him.

We live in townhomes and there is a playground and thats the only place they interact. I am always there but the other kids mom is never around, so i cant speak to here.

There has even been other parents that have told the 5 year old boy to be nice and stuff like that. I have also talked to the boy, he he starts yelling at me stuff like "i dont care what u say" or "what r u gonna do about it".

I dont know who his parents r and I dont know what to do anymore. Any advice?