Given I played Smash with a younger brother growing up and know how stubborn we both were about it back then, I can relate. I do agree with you, and Smash is a game of adaptability, but some people just don't want to play it that way, especially if he's young, and honestly, that's fine. He just wants to be Little Mac and that's how he has fun. However, given you're playing casually with your brother, surely you can both reach a compromise to keep it balanced for you, as well. Take turns choosing stages, play your mains on stages he chooses, and different characters you're not good with in stages you pick, so on. Find ways of balancing. Have a rule where you can't use the same character two or three times in a row, and have him make up something silly. Pick each other's characters every 3 matches, or the last match's loser gets to pick their opponent's character. Agree to set the stages to random if it's a real issue, then he'll have to play the hand he's given. Make it up as you go. Fun should be the priority over competition, especially between siblings. Find interesting ways you can agree to make it fun for both of you, it's worth it to try.