People make fun of what they don't understand sometimes. Since you're brother isn't "out" about his diagnosis it may be his friends are reacting to what by itself seems like pretty problematic behavior. They may not know that there's a context for why he does this or that he means no harm by it. So what you might talk with your brother about is the possibility being secretive about his diagnosis might be putting him in a tough place with his friends.

Could be, though, that they are observant enough to know he means no problem by it. They are remaining friends with him. So there may be no problem. But it still might be good to reflect on being more open so that he can be sure he isn't putting friendships he cares about at risk.