My kids are 4.5 and 2.5 years old and both are in preschool. I agree with what others have said. Hitting and being aggressive is not acceptable. However, I do teach my kids that they can and should defend themselves. BUT that is ONLY after they have told the other kid to stop, walked away, and told the teacher. If it continues repeatedly and the kid STILL keeps coming after them, I feel that my kids have the right to defend themselves with whatever means necessary. But again, this is AFTER they have tried other means to get away from the other child. Both my kids just naturally have a "3 Strikes and You're Out" system. They will both let the other kid attack, hit, or pursue them 2 times. Two times my kids will either tell the other kid "No" and walk away or come and get me or the teacher. On the third attack, my kids will hit or push back and then come and get me or the teacher.
If my kid is the aggressor in the situation, they will get an immediate Time Out and then either DH or myself will explain why hitting/being aggressive is wrong and if possible we make our kids apologize to the other kid.
If our kid is on the receiving end and eventually defends themself, we pull them aside and tell them that we are proud of them for trying to get out of the situation the first two times, and that although we don't like when they hit other kids, we know that this time they were defending themselves from a kid that wouldn't leave them alone.
The way we feel is that, this is real life training for them. They are going to come across aggressive people in their lives. They need to learn, even at this young of an age how to handle themselves in these types of situations. I won't always be there to take care of the situation for them.
I also agree if his preschool is not handling the situation to your satisfaction, move him to another school. Our school immediately deals with the situation. It's a smaller school, so all the teachers know all the kids and the parents. Everyday when I pick up my kids I ask how my kids did at school. The teachers tell me everything, good or bad. My son has been involved, both on the giving and receiving end of aggressive behavior this year, and every time the teacher told me about it and I got a note sent home about it. They immediately put the offending kid(s) on Time Out and then explain to them that their behavior is unacceptable.
I think general give and take type pushing and wrestling is normal for this age, especially boys. That doesn't mean that it's acceptable behavior, but it is normal. Even hitting and "fighting" are normal, but definitely not acceptable. From what I have learned from other Moms of grown sons and the preschool teachers, is that between 3-5 is when boys experience their first surge of testosterone. That's why they get aggressive and go through periods of high energy, followed by periods of low energy, and periods of pure unexplainable frustration. Kind of like a mild version of puberty.
With 2 year olds, they are just testing the boundaries. Trying to see what their bodies can do, how others respond to what they are doing, what they can get away with, etc. I don't think your son is intentionally trying to be mean to another kid or vice versa at this age. But the actions are still not acceptable. So just keep reinforcing that being aggressive is not acceptable. Defending himself is good, but only at the appropriate times and after he has tried all other means of diffusing the situation. And true, he won't get it at this age, but the key is to keep reinforcing your rules so that as he gets older, he will abide by it. That's what all the older Moms of boys I know keep telling me anyways.
I also show up and sit in on my kids classes every so often. a) to check up on the teacher/staff/other kids and b) to check up on my kids and also to show them support.
Also I come from a background of martial arts. We were taught to never be the aggressor, but the defender. Let it go, let it go, let it go, but when things start to get out of hand, or someone is really in danger, step up and deal with it in the safest way possible, even if that means becoming the aggressor. My son also took Tae Kwon Do lessons for a year from age 3 to age 4. His teacher followed the same principles. I think having that extra reinforcement of our rules from his teacher really helped him to understand what to do in those situations.