frieda, it sounds as if you have tried all the approaches I can think of to find the root cause of your son's fighting, so I can't add much there.
I would say IME it's important to keep stressing how he should behave - give him something specific and simple to aim for. Ask him to 'tell the teacher' if he feels some child is annoying him. Someone here once gave me great positive advice - tell your child to 'use his words' rather than keep telling him it's bad to hit. It's like a mantra. Every time your son is involved in an incident, talk it over and ask him if he 'used his words.'
My oldest son went through a similar hitting stage. It really helped me to find out all the background info. Teachers, as you know, don't always have time to tell you, but do ask, and then ask your son.
Do the incidents happen in the morning or afternoon mostly? could your son be acting up when he's tired?
Do they involve the same group of children? if so can your son find other friends to play with (perhaps invite one over for tea or ask the nursery workers to encourage your son to leave the gang)
Does your son playfight or have very boisterous games which then lead on to hitting? If so, can the playfighting stage be tackled more so the hitting stage is avoided?
Obviously the nursery will be doing this sort of thing already, but it helps to know what they are doing.
I think my son was a little immature and easily excited. He couldn't control his emotions sometimes and would hit out rather than speaking out. He did grow out of this, to give you hope. He's 8 now and is not a fighter.
Also, you say that you have witnessed your son winding up other children first. It might be that he is retaliating - are you sure the other child did not wind him up a few minutes earlier when you were not around? It could be part of an ongoing sitiuation - even a game gone wrong. So, have you thought about staying in nursery with your son for a morning to see how all the children gell togther? that might be worth doing, too. And explain why you are doing this to your son, so he knows you take this sort of behaviour very serioiusly.
Hope you find things calm down soon.