Maybe you'll just have to keep the higher difficulty stuff for when he's not around then? Depends, if you still enjoy the game on normal difficulty then that's probably the best solution. Tough situation though. You could ask him if he wants to try higher difficulty and if he says yes but then starts shouting just say "Hey, it's on hard, no big deal, don't get worked up about it. We'll get there eventually, it's all part of the challenge." You'll know better whether that'll fly or not though. Just get a big @matt pascual motivational poster with #StayPosi written on it and hang it over the TV.
A poster of me is clearly the best option but like other people have said it sounds like you two just enjoy games in a different way and trying to force it will just lead to more frustration.