I haven't read all the responses here so apologies if I'm repeating myself but just wanted to say that my ds is a bit of a bully. I'm quite a strong personality and do play a lot of rough and tumble games with him as I like boys to be boys but at the same time I don't encourage in any way him hitting, biting or doing anything else to his other friends.
Ds is 2.5 and he had a bad stage of hitting all his friends just before he turned 2. There was no reason for this at all (he often used hard or sharp toys to do it with too) but just acts of pure agression against his friends and I was horrified. I tried a whole range of methods from speaking very firmly to him on his level, taking him off to his room, getting him to say sorry and things like that. Nothing seemed to really work. He then grew out of that phase but when his little sister came along did revert back to it for a couple of weeks. Funnily enough he has never done it to his sister but only his friends who are pretty much his age.
He has just started biting when he has never been a biter before and is becoming very agressive again. The first time I thought it was a stage he was going through but this time I have realised that it is actually due to the change in his environment that is causing it. We have had a lot of visitors and his new nanny has just started etc so he is really lashing out and I tend to cop the majority of his hitting, head butting, biting and so on.
It doesn't seem to matter how much I tell him I love him, cuddle him or anything else he is still going through it. To be honest even though it is my job to discipline him when he does these things at the same time I actually appreciate it when another mum/dad does it as he certainly listens to them much more than me. The problem is that most parents won't do this as they don't want to step on my toes (as such) and with my friends I quite often have to ask them to do it if he has acted out towards them or their children and of all people they should know that I don't actually mind.
Anyway, the long and the short of it is please also think about the little boy as there may be a reason why he is doing this. It is unfortunate that it was taken out on your child as opposed to maybe his mum or dad (who can handle the pain etc) but also please do as the others have suggested and reprimand him in your own way so he knows that it is unacceptable. If his mother challenges you about what you say or do to her son then don't be afraid to let her know what you have said, how you have said it and why you said it, therefore, because she wasn't there at the time or didn't do it herself and if it isn't done straightaway then the impact on her child will be lost.
As the others have said, don't give up going and also teach your son how to deal with things like this as he needs those skills himself for world he enters himself, eg school, work etc.
So please also remember that