Listen to your parents. They sound wise.
You don't need to talk to your ex about anything. I wish you'd stop talking to him! ;-)
A solution might be just to send your stepson a small gift and a card/letter in the mail with a message in it saying you'd love to see him when things calm down; after the holidays, after the legal issues are settled, for a special get together just the two of you.
I think he will appreciate your honesty. Fact is that he is your step son and yes, unfortunately, he is turning into collateral damage because your STBXAH is using him as a pawn.
But you don't have to participate. In fact, if you do participate. You can walk away from the table.
In the end, the best you can hope is to save yourself and your daughter a lot of pain and heart ache. That is your responsibility.
And did I say already, that your parents sound wise and you should listen to them?