Changing schools often solves the problem, if only temporarily. I personally don't think it's a good idea to throw your cash to a foreign academy, though. As someone whose dad shelled thousands to send me and my little brother through the same school for thirteen years, I don't really think it's worth it.
Just tell your son to fight back. Better than pussifying him through punishment for defending himself. In 4th-6th grade I had a bully problem. The thing that really tripped me up was that a handful of the boys I had to deal with were shorter and smaller than I was. Yet they'd push me around just the same. I fought back, not even that aggressively, and then my teacher got me in serious trouble. I was guilty by default, no matter what I said, and it just sucked. My mother sided with the school 90% of the time, and my father (divorced) had his usual stoic, passive aggressive tone about the matter (I somehow didn't figure out that what he meant was "I don't give a ****," but I didn't learn ***** dialect till later.) Looking back, I should've just kept hitting those boys and gotten expelled. Would've gotten mom mad, for sure, but hey, at least my point would've gotten across much more effectively. If your son gets expelled for hitting students who hit him? Then great! He doesn't have to go to school with them anymore.
I remember one afternoon rather vividly. I don't even remember if I did anything on that particular day, or the day before. But the afternoon is what I remember. And it's something that no upper elementary child should have to experience. Ever. My mother always liked to use scare tactics and matter-of-fact statements ("I am so angry," etc) and on that day she made such a statement. We talked to my principal, and it was in a relatively common area, and my mom made some statement about how badly she wanted to punish me after my principal told her that my day of suspension would be the day after tomorrow due to a field trip I was skipping, and there I was on the verge of tears. And she didn't give half a ****. When we got home, she said even more things to me that directly attacked my self-esteem. "You shouldn't even be in Ninjutsu" (a martial arts class I took after school) she told me. That exact line is one I'll never forget. I didn't know any better at the time, but what she was doing was emasculating and invalidating her boy which should soon become a young man. And she spanked me afterwards and all I did was take it. I should've started attacking my mother and beating her with all I had that day. That's what I honestly believe. Not kidding.
The hard pill to swallow about private schools is that they aren't this bubble you can protect your child from weed, alcohol, weapons etc. with. Every secondary school has cliques, stoners, football status (my school's ape****-over sport of obsession was actually basketball since we didn't have a football team - at least not without a raise of our tuition), weapons, and ****ty teachers.
Public schools are "free of cost," yet the public school system in America is more or less a big arm of the liberal agenda funded by tax dollars.
Private schools are touted as "safer" and having "better curriciulum," yet disembowel your wallet.
Homeschool only works if one or both parents is able to teach it. It can't work in most cases if both parents are working, unless you happen to outsource the homeschooling to another adult. That said though, my main problem with homeschooling is that I will want my child to be exposed to all sorts of people that he loves and hates. If you homeschool, you can't have your child make friends. I know it'd suck for ME if MY only classmate was my little brother.
