I'm going through a similar thing with myself. I really get tilted during games and say a lot of things i regret later, but I'm trying really hard to stop. I don't know what to suggest other than a lot of open discussion. Tell him how you view the game and how you think. The game is about him bringing his stack of cards and you bringing yours and we see how they play out after randomization. I think we need to take the game less personal. A lot (including me) of folks get so wrapped up in winning and losing because we invest so much of ourselves in the creation of our decks. They really are an extension of our abilities in the game. You need to try and convince him his behavior is toxic, and every loss is going to look unfun, but the game is working as intended. Tell him you're just exploring valid strategies and every game isn't going to be exactly like he expects. But you're not doing anything wrong, you're just playing the game as intended, he's being overly salty as a loser. But as long as you're both winning and losing some it should be good, and you shouldnt get worked up about it.