I think you are doing the right thing....I know that with this sort of thing it is really best to stay out of all of it...(at least this is what i have found)
My 6 yr old DD had the same problem at her school with a few of her friends but i found they always sorted it out between themselves in the end....
Maybe just suggest to him gently that he needs to find some other people to play with too?? I dont know whether your DS would have anyone else to play with but if he does maybe it would be better if he was playing with the others and looking like he really wasnt worried by x's behaviour towards him...
I really think that they will sort it out themselves in time though....
((((hugs))))) it is so hard when your kids argue and you want to defend them but know that it isnt the right thing to do .....