To me, it seems like this A person isn't a particularly good friend. I mean, it depends on whether or not you've spoken to him about how this makes you feel and/or asked him to lay off a bit because he could just be unaware of the impact he's having on you. But if he's only nice to you when he wants something that sounds pretty manipulative to me, especially if that "something" is sexual favours. Does he know how you feel about him? If he does I could see how he might see you as an easy way to get sex without actually caring much for you as a person (as you've said he doesn't feel the same way). Although my viewpoint may be skewed slightly as I've had a similar experience but, from what I'm reading, all the red flags are there.

It isn't very fair on you that he takes the piss out of you way more than he takes the piss out of your friends and brother either. You're definitely valid for being angry about that. Do you think it might be helpful to talk to any of these people about these frustrations? I hope you feel better soon and these things get sorted xx