Hmmm tough one. I always approach things from an "honesty is the best policy". I would tell your son the truth in a limited detail, matter of fact way, and in terms that are appropriate for his age.
One thing I jsut thought of: Are you inviting everyone else in the class 'cause if so that makes it even tougher, IMO. If your son is excited he will want talk about the party and regardless of what you think of the dad, the boy will be hurt. If you ask your son to keep the party private and not talk about it he will feel guilty. If he has a reason that makes sense *to him* it might make the situation easier?
One thing I jsut thought of: Are you inviting everyone else in the class 'cause if so that makes it even tougher, IMO. If your son is excited he will want talk about the party and regardless of what you think of the dad, the boy will be hurt. If you ask your son to keep the party private and not talk about it he will feel guilty. If he has a reason that makes sense *to him* it might make the situation easier?