I'm sorry you're going through this - divorce is never fun. It's really nice that your parents are being such adults about it, that definitely makes things easier.
I am also the "most OK" one in my family with my parents' divorce (and subsequent recouplings).
I would probably ask him straigh out what he plans to bring up at this meeting. If it's what you suspect I would try to talk him out of it, I have had no problems telling my sibs if I think they're being unfair or ridiculous about something related to either of my parents. If that didn't work I'd rat him out to my parents
so they would have a heads up and could decide how to deal with it, and if they decide to go through with his meeting I'd go, and sit beside your parents and support them.
ETA How specifically to support them would probably depend on context...I would maybe just bring up things like how many sacrafices they have made in their life for him, how they have supported him through his decisions, how they are adults and capable of making these types of decisions, etc.