I know it''s hard, but letting your son make his own decision is the right thing to do.
If your stbx is living with someone unless you have proof you don''t know what that persons monetary situation is. The fact that her child goes to private school could be be paid for by her/her ex as could the holiday.
He has admitted adultery and we are divorcing on these grounds, so I know they are living together. The OW has no contact with her son''s father so I know that the father isn''t paying for his private school fees. And I know that my STBX is paying all the living costs for her (extended) family in their new home. It''s the whole thing of him choosing to provide them with a decent life while we go without that makes this whole holiday thing so difficult to deal with ...I''m still glad I asked about it and still think your answers are right. His behaviour is unfair, but that''s a separate matter to the holiday.