I agree with KyleKat - you have a broken relationship - adding more people isn't going to make it any better.
If he wants to make things work with you then he needs to put what energy he has (which may well be less than he used to have - we're none of us getting any younger!) into your relationship, rather than using it to find others. If he really has given up on it, then you both need to recognise that and take the necessary steps.
I also agree that this has nothing to do with polyamory - the relationship isn't on the rocks because of poly, it's because of your relationship.
"Listen, or your tongue will make you deaf." - Native American Proverb