I don't think seven year old boys are all just a product of their parents, but they are also at a very impressionable age where they actually need teaching right from wrong. They run in packs in the playground, and if they see that hurting someone else or calling them a name works, they carry on using their new-found power, often with others joining in. They need stopping in their tracks, and at that age, in most cases, being hauled up by the school and your parents and being sat on very very hard works (not literally, obviously!) He needs to be shown that what he has done is wrong, doesn't work, gets you into loads of trouble and makes your own life a misery.
Worm's advice is crap, and only likely to end up in you getting into a verbal or physical fight, as most people will defend their children even if they are in the wrong (or they may be quite bullying themselves).
If direct action is needed, then it should be your child sticking up for themselves against this boy. That doesn't sound possible, so you need to create an alliance, of you, your son and the school taking him seriously and defending him against this bully. That is much more powerful than creating an out of school feud you are unlikely to win (unless you really are the hardest mum in the playground).
I also felt very angry towards the little shit who hurt my child, but now I can see that actually, although he did the wrong thing, he is not actually a little shit but a seven year old boy who needed very firm direction, and he's not all bad at all. Even my children don't bear a grudge and like playing with him now. This is because I didn't let the situation develop but leapt on it like a ton of bricks. Whoever said go into the school on a daily basis til it is sorted is spot on.