delericho
Legend
What do I do?
When he's DMing, he can institute whatever house rules he wants. If you want to play, you should respect them. (Obviously, if the house rules are sufficiently bad that they make you not want to play, you know what to do!)
When you're DMing...
He's taking it to a point where he is adding his constitution to death saves when I DM, how am I suppose to convince him?
Don't. Tell him he's wrong, point to the rule in the text, and advise him that if he doesn't play by the rules of the game, he's not welcome. He's free to think the rule is as stupid as he likes, but that doesn't matter - if he wants to play at your table, he has to respect the rules you're using.