that's his decision and his only. you can't decide in his place: the best thing you could do is drag him to a tourney, do him a favor and pay his entry fee, throw him in the pit and watch how he perfoms. after that, he'll either leave for good or his competitive spirit wil re-kindle.
the "travel to tourneys/fests, meet new people to play with" part is essential for competitive brawl methinks, otherwise there's little to no growth and/or interest.
I won't try to persuade your brother, if he's reading: I'll just point to him the path he should take to make brawl "not boring".
I would suggest online too, but that depends on your brother's opinion about it.