K.K.
ok Brushing is important, BUT there are other ways to promote dental health. Mainly Diet, If you have pop/sports drinks/ koolaid in your house and allow him to drink it, then you are harming his teeth just as much. I'd tell him, either he brushes or it's Water only.
Apples, celery those sorts of things are great for helping to take care of teeth.
I'd be tough, either he brushes or there is nothing sweet and sticky to eat or drink.
Ask your dentist where to get the purple tablets that show were he missed brushing. I think drugstores have them.
The party thing is tough, We don't have a party each year, but if you do know the other kids are having one and just aren't inviting our kids then it does hurt. I guess i would do what i could to help your kids each make one special friend who would invite them in return. focusing on true friends instead of just inviting everyone you know.