You need to be a bit careful approaching parents. Sometimes it's better to find out if other kids have had the same problems, and if their parents have ever spoken to the bully's parents.
My 9 year old son was being bullied by a lad of 13 who lives near us. The bully is on anti-depressants and receives counselling for depression. I'd let it go for a long time, as the bullying wasn't that serious and my lad just shrugged it off most of the time.
This lad has bullied his way through most of the younger local kids, but nothing gets done about it because his father is in the police and gets very, very aggressive when approached about the behaviour of his son, even when the person approaching him is female. He is also 'with drink' on a lot of these occasions and has assaulted people in the past.
Both parents are also quite happy to lie about their son's behaviour.
One day he gave my young lad a right kicking. Holding him down and punching him in the face repeatedly. I'd had enough. I went up to speak to the parents, who claimed that they'd seen everything and that my son started it.
I asked them why, if they'd seen everything, they hadn't stepped in and stopped it before my son had received facial injuries. They had no answer, except for the father to become very aggressive towards me, because, once again, they were lying to protect their son and had seen nothing.
I told him very calmly that I wouldn't accept any aggressive behaviour from either him, towards me, or his son, towards my lad, in the future.
His response was "or else what"? So I told him that I blamed him and his wife for failing to bring their son up with a proper sense of what was right and wrong, and that I would be holding him responsible for his son's actions in the future.
I explained to him that if his son ever gave my lad a similar beating, I'd be round to dole out the same to him. No talking, no discussions, as it was obvious that he was beyond reasoning with.
We have had no trouble since.
They will be moving soon anyway, as he got convicted of drink driving by the local constabulary recently and lost his job as a result. So much for his 'mates' in the police!
Not an ideal solution, but sometimes there really is no reasoning with people.
I hope it doesn't come to this in the OP's case.