This is the best answer.
This is a boundary I would definitely not cross. Not only would this fuck up my mental health, but there would definitely be some kind of argument that would happen during the trip that would just make the whole experience regrettable and worsen the situation between all parties.
If I were OP I would definitely say no and instead explain to the boy why it’s not possible.
Well he obviously wants to spend time with his dad, which is great.
Maybe OP can find other activities to do on vacation. Like renting a cabin or cottage somewhere near a lake or river. I know I loved to go on fishing trips with my dad. Or doing campfires and roasting marshmallows, etc. I miss the 1on1 time we spent together.