I may be completely off base here but it honestly sounds like hes envious of the relationship you have with your OSO (btw if i get any of lingo on any of my posts wrong please correct me). As a temporary measure you may want to try limiting the interaction you have with your OSO around him. I know that you absolutely should not have to do this but as a temporary fix it may help more than you think. You or someone else should also tell him that he won't find happiness if he is looking for it, if he really wants somebody else to like him he needs to just relax and focus on being him, not being him AT somebody else; it will make him come off as less desperate and he will have a lot more luck.
Lucy can tell you, that only when we work in, do things begin to work out