He is not getting his needs met some how it sounds like and is turning it around on this guy as a result. That is fine in a moment, but when it turns into a constant it can get tiring, completely unappealing and counter productive... You might want to tell him that as I would think eventually you will just plain get fed up with his disrespect of the other man you love. It will eventually show you that HE is actually the one that is not worthy of you and the other man is.
If he really wants to do the work required for you to be poly then he needs to look at the other man differently. He needs to start thinking of this guy as simply different rather than better or worse than him.
It's your choice who you hang out with and who you love. Love should be respected and have no bounds of looks and character... it's really not his right to make that call for you.
If he were smart he would be welcoming him and getting to know him as a possible metamour if he wants to keep you... really show that he is worthy of you as well, instead of just saying it. If it should be that this new guy isn't really for you, then his hosting of this guy will make that shine through. Not only that, it will make him look WAY better in comparison. If he continues on the path he is on, then you will not notice that he perhaps isn't for you and it will last way longer....
what does he have to lose? He could be charming and a good host and gain an amazing metamour, or do the same and have a deeper relationship with you as a result. it's all win win if he chooses.
Here is a link I started on "relationship theory" on being a good host.... hope it helps.