K.P.
Wow- I would RSVP with a "no" for a ocuple of reasons:
1. He said he didn't want to go several times so he probably really doesn't enjoy this other child.
2. If he's calling the boy "stinky pig" you really don't want that phrase used at the party
3. Your son may benefit from learning a little lesson here... he made a choice adn now he needs tolive with it.
As for the kid stinking, I would talk to the preschool teacher about the whole thing and see what she says. Even if the child doesn't "smell" she should be aware of the name-calling that is going on in the classroom. Bring her attention to it (if she isn't already aware) and ask her what words she is using to stop the behavior so you can reinforce it at home (hint: this will let the teacher know that you are keeping an eye on this and she should too!). If the child really does have an off-putting odor, you may want to encourage the teacher to bring it up with the parent.