I'm going to go though the proper channels and also give my son free reign to defend himself. It's not that Tyler won't hit, he's at the stage where he thinks every school rule is sacred to him. When I have asked him why he did not hit the boy back before he always says it's because he does not want to get into trouble. Suppose I need to convince him it's not wrong to hit someone if they already did it to you first.
It'll be much more effective talking to the head teacher if you have a clear idea of what you want from the discussion, I'd suggest the following:
Make a formal appointment with the head or deputy head (and don't bother with a year head etc) to discuss the issues: this signifies to them that you are serious about it and allows you to ensure that a record is kept of the discussion. Just "having a word" gives them an excuse to ignore it or "forget" the discussion.
Obtain a copy of the anti-bullying policy for the school. This will set out very clearly what they should be doing (but very often don't do) and can be a useful stick for you to prod them with.
Be clear about what you expect to happen (the bullying stops immediately) and also be very clear that if you son continues to be bullied by this child after your meeting you will want a further meeting to discuss why it hasn't stopped and what the school intends to do about it (talk to the parents etc).
Your language is important: the school staff will more than likely use terms such as "boisterous play" or similar- if you are framing the discussion in terms of "physical assault" they will sit up and take notice. Also make it clear to them that you have told your son
he is allowed to defend himself and that you do not expect him to be punished if this happens - the school should instead be punishing the bully for instigating it.
I wouldn't raise it with the kids parents in the first instance, no-one likes hearing that Daddy's little hero is actually a spiteful little **** so leave it to the school staff to do that job. Might be worth asking around the other parents to see if anyone else is being picked on by the same kid though (your son may well know), if they follow the same process I've outlined above then all of a sudden the school has got a big problem that needs dealing with quickly.
Let us know how you get on.