Its not you who is making the decision to not send their son, its theirs. The longer you accommodate this, the longer they will let you, and be excused from having their own frank discussion about what is best for their son. If they decide to deprive him of family for the reason that you are there then they have to live with the guilt, not you. Your Son on the other hand will probably want you there, so you don't deprive him of this enjoyment because some adults can't be adult about it. Good luck, oh and enjoy the party!