It sounds like this falls in the realm of things you cannot change (other people). Sounds like your ex has full custody of your son. As the person with custody, she can choose who can see him. Also with her being in another country, I'm not sure what the legal situation is. But I don't think there is a legal way to keep your brother from seeing your son.

There is also know way to get your brother to stop talking about you. If you live with your parents, you can have a talk with them about boundaries. They might agree not to let your brother visit; then again they might not.

In situations like this, I remember the serenity prayer: "Grant me the serenity to accept the things I cannot change." The one thing you can change about it is your reaction. Maybe your brother likes getting you angry. If you can hide your anger and even act like you don't care, he may leave you alone. I know my brother loves a fight and gets frustrated when I refuse to fight.