I don't know, I mean I can see why you'd take those comments like that but I think it's because it's a sensitive subject and I think he's actually trying to show you he doesn't care that you've put weight on.
Hair extensions- I agree with him, they give me the creeps too to be honest and can sometimes look a bit...grubby...nothing against the people wearing them, just my opinion. Also, of course a different hair cut can make you look slimmer or just different around the face and neck.
Sofa, sounds like him just making a joke, he knows you're sensitive and was trying to lighten it a bit
Chocolate remark, I'd probably have said fine I'll eat yours too then if you don't want one, but again, he's probably trying to help you, would you rather he kept offering you chocolate? The reverse would be he's trying to sabotage your diet.
Only you really know him, if he's not nasty or controlling elsewhere I'd think this was a clumsy way of trying to make a joke out of it to show you he's not bothered about it, but he knows you are, so he's just trying to make light of it and support your diet. Tell him to stop with the jokes, you don't find them funny, and see if he does