I recommend always starting a dialogue with the person doing the offending, try to get them to understand why the things they are saying and doing are harmful. Try to put things into perspective for him - he is thin privileged and unable to relate, ignorant to a healthy way of addressing body image. Help him to learn, if he is receptive, and explain how you feel.
Try to keep as level-headed and calm as possible and back up your arguments with facts and experiences that hopefully he’ll be able to understand. It is possible he won’t want to hear it and will completely reject anything you try to speak with him about. This is not your fault. You can’t control the thoughts and actions of others but you can control how their thoughts and actions affect you by minimizing the impact.
If he’s not listening to you, then it’s your turn to tune him out. Don’t react to fat shaming comments. Leave the room. Don’t give him a chance to fight you about it or make you feel like shit. You don’t have to accept it. It’s simply not worth it, and you deserve better than to be talked to and treated in that way. He is your brother, and you love him, but that doesn’t make him right.
When people in my family or friends that I am close to disregard my feelings about these things, the best thing I find I can do is to simply just…ignore. Let it roll off your shoulders as best you can. Dwelling on it will not improve it.
I hope he with be receptive and open to discussing these things with you. If you need any help, there is a resources link at the right side of my blog, and the “resources” and “quotes” tags are also good sources of info and facts on fat health and body image. Good luck. <3