Is it permissible to boycott a brother who demands to be served while he is capable of serving himself, and who resorts to boycotting if his demand is refused?

You are not obligated to serve your brother or to do what he commands you, but doing so out of kindness and beneficence towards him will be rewarded. He has no right to boycott you if you refuse his commands, because you have not committed a sin. If he boycotts you, do not boycott him in return; rather, greet him with peace (salaam), and the sin will be upon him.