Being a prepubescent boy sucks. Having an older brother REALLY sucks, because all older brothers treat their little brothers like shit.
Do the reverse of what you would think you should do, seriously. Give him tasks that require him to be responsible for his actions, with clear cut consequences for success/failure, and leave him to the work, don't hover. Give him an opportunity to prove himself, some way to start working towards adulthood that sets him apart from the older kid, and something that gets him positive attention.
C'mon man, think like a kid. Remember what it's like being a boy that age, what does cockiness really cover for? Insecurity, lack of confidence. And being a loose cannon is usually a bad way to get parental attention, because any attention is better than being lost in the shuffle.
Translation: SB is a dead beat daddy.