So, my boyfriend lives with his Mom, and his brother.
If he's your brother, then aren't "his [m]om" and "his brother" also your mom and your brother? Or is this a half-brother?
Said brother decided he no longer wanted to live with them, is under the age of 18, and wants to get emancipated.
Ok...how old is he and in what state does he live?
I'll mention that I'm in Canada right now
Where you live makes no difference. What matters is where your brother lives.
My question is, what can we do?
I'm sure you CAN DO any number of things.
We (the mom and brother) want him to not be able to legally live on his own. And we also would like to sort out which items he owns, and what he doesn't own.
In most (if not all) U.S. states, any property "owned" by a minor is really owned by the minor's parents. As far as your brother not living on his own, obviously, his (your?) mother can (and probably should) not succumb to the blackmail and not sign off on the emancipation. Note that, even if she does sign off on it, in order to be emancipated, your brother will still need to demonstrate to the court that he has the ability to support himself without assistance from anyone else.