The kid is only 16. I hope he is able to talk to one or both of his parents and they can actively help him enforce his boundaries. I know at his age, your friends can feel so much more important than family but really, a good supportive family could make all the difference here. If his parents are the type of people he can go to and who can help him understand that his 'friends' are trying to break through his normal and natural boundaries then they can help him enforce them. And help him through any backlash he may get from these terrible friends.

In fact maybe his parents could talk to his 'friends' parents if they know each other. Because the friends parents might also be able to reign in some of this shit and make the friends know how awful their actions are or support the two friends who recognise how inappropriate the whole thing is to support this boy more openly.

And to get the other kids to realise how utterly awful they are being. I know if my child was trying to push another kid to date a third when they weren't interested, I would want to know so I could make them understand that they needed to stop. That the mental health issues of the transkid were not the responsibility of a another child to ease. That it wouldn't even be possible even if the OP was attracted to the transkid. Nobody can fix someone else's issues like that and all the friends are doing is making everything worse by creating false hope and eventual resentment.