Yes, speak to the school. Start with his teacher then the head. If the situation doesnt improve, then go to the next governors meeting and bring it up there - I can assure you it will be taken VERY seriously.
Its also worth speaking to the dinner ladies, as they are often aware who the "troublemakers" are and they will keep a special eye on your son.
Its a very hard call about what a child should do to combat bullying. My DH was severely bullied throughout his schooling and consequently he hated every second. He wanted me to teach our DD at home (she has just started Reception) but I refused, feeling she needed to mix, and the fact she loved Nursery so much. I caught DH telling her she must "stand up for herself" at school and "fight back if nessacary". I was quite shocked, but he said if he had done that from day one, the bullies would have left him alone.
Its not something I am comfortable with at all, but I CAN see both sides of the coin, so to speak. It doesnt seem right that your poor son is being victimised and then punished for standing up for himself. However, when the other lads go home and say "XXX hit me today" the mothers probably have NO IDEA that their precious sons are the instigators of bullying and would expect action against the other child.
It seems there are unresolved issues here that need dealing with, and in my opinion, it is the schools responsibility to you, your son and the other pupils involved. Dont be fobbed off by "words", demand action and some positive anti bullying messages from the school. If the school has a prospectus, see what "measures" they promote to tackle bullying. If these are not happening, then ask why.
I wish you and your son every success. Bullying is one of the worst things to have to deal with as a parent. My DH has never forgiven his parents for "not believing how bad it was" (it wasnt the same immotive issue that it is now) and although he now has a great career, his own business (and earns tons more than his "bullies" now do

) it has left a scar on him.
Once again, good luck and...
Allie