I'd try to tiptoe around the political issues, avoid mentioning it as much as possible and certainly not dwelling on it. You can hope and pray for his safety and prompt return home without saying much of anything either for or against his role as a Marine.

You say you don't know your cousin very intimately, so you probably are not well enough acquainted with her son to know his (other) interests. Too bad; if you could discuss music, educational plans, sports, anything, that would probably be the most comfortable both for you and for him.