I think, to a certain point, that you should let your son defend himself. Pushing and hitting not the answer, Teach your son to stand up for himself with his words and actions. I am currently reading a book "Raising a Self-confident child" by Magda Gerber. It goes over this topic. First it suggests speaking to the aggressor say thinks like: you cannot hit/bite, doing so hurts, that upset so and so, see how that made so and so feel, you don't want so and so to feel that way do you... The book goes into more detail. It also suggest a "shadow" person to keep an eye on the aggressor at all times to stop the behavior before it starts. However, it does suggest letting the problems to occur only to the point that it becomes dangerous. Meaning that children have to be given a chance to have a certain amount of conflict to learn problem-solving skills. Social skills, interacting with peers, and standing up for oneself. Secondly, the book suggests validating the feelings of the "victim"... without making them a victim. It says not to go as far as one might be made to feel like a true victim but just validate. Saying things like: I know that hurt, I am sorry that you got hit/bit, ending it quickly. Overreacting vs. Under-reacting or what she calls "doing less." Work with your Day Care provider. What are their policies, how do they act/re-act, what do they say to the children. Suggest a "shadow," someone to watch over this child and intervene minimally (only to offer suggestions to the children involved on how to act appropriately in this situation. Such as: To the aggressor: We do not hit/bite, if you are upset you could ...fill in the blank... To the "victim:" Hopefully before the actions occur: Johnny doesn't feel like sharing/playing right now maybe we could go over here and... Re-directing attention elsewhere.) Ask if they have talked to this child's parents (are they working on it at home, are they going about it the right way, or is he just getting yelled at,) see if they would be willing to try some techniques such as tying a plastic doughnut (the kind from the plastic stacking toys) around his neck/wrist for bitting... teaching the child to use this during aggressive behaviors. Nobody wants their child to be the one who is getting bit. Teach your son to stand up for himself. Not by bitting or hitting but by expressing his feelings. I don't like when you bite/hit. I will not play with you. Walk away. Maybe if more of the children begin to do this this other child will realize if he wants to play, if he wants friends, he will have to behave in a certain manor, or have no playmates. I get that this child's actions are bothersome to say the least. But keep in mind that children learn socially acceptable behaviors at different ages. It is normal. If all else fails I would first conclude that this child's parents are not teaching their child socially acceptable behavior. Secondly, I would not consider, necessarily, switching Day Cares. You like it, your son likes iit. This Day Care has an obligation to you, to keep your son safe. He is not the problem. The other child should be asked to leave. Ask for a meeting with the family and the Day Care.
On another note, keep in mind that this child may have older siblings that are modeling this behavior. Maybe he is the one being picked on at home. You do not know what's going on at home. It could be much worse. Do you know this family? Maybe you could make an effort, if for no other reason than to just get a feel for what's going on and/or how they are parenting.... before you demand a meeting with them and the Day Care. Maybe it is a young or single mother who does not know what or how to stop the behavior. See if you can find out more. Maybe you could share your knowledge, maybe you could help them help their child.