Hi A.,
I have some insightful features on bullying on my blog at http://www.tristansepinion.blogspot.com - look under "On Topic" (right sidebar, halfway down - under my new HERO, Walter Olson :-).
You are rightfully concerned - whether this other boy's behavior impacts your son or influences him as he grows from 6 to 16. You can't control who he chooses for friends at 16 so now is your chance to help him gain the skills and self esteem to identify behaviors that are not positive in his life - to choose friends wisely.
I would speak with the mom - and I don't care if her attitude is "boys will be boys" - because, when her son is in YOUR home, this "cute" little slogan only has to work if you let it. I would let her know that I respect her perspective but, in my home, the motto is "boys will have manners and respect for others."
Since her son has problems adjusting to that rule and a party is a situation where you cannot be there to police every child's behavior single-handedly (while juggling cake, games - we know the drill!), let her know that, if he'd like to come, he's welcome - but she must ALSO attend and be responsible for parenting him so that you do not have to.
The reality is, if any other child were to be injured while in your home, you would be liable and feel horrible, too!
If she cares about your concerns and has respect for YOU, she will show her respect by honoring your request without issue - she'll understand.
My direct two cents (from an old fashioned girl) - there are times when others you like have horrible children and times when others you don't much like have lovely ones. Either way, it's alltogether your right, privelege and responsibility to consider what the best choice is for your family. Sometimes, you really do have to throw the proverbial baby out with the bath water. If you have taken the time to express your concerns and there is no interest in responding cooperatively, on you / your family's behalf, then you may want to consider such an option.
If this helps, great...if not, toss it! I wish you a wonderful party without incident!
Warmest Regards,
T. B.