I think lisbeth has a good point. Does this type of controlling behavior spill over into other areas, or just this area?
If this behavior spills over, then you are simply dealing with a controlling person and you are going to have issues with that no matter what.
If this behavior is isolated and only showing up with reference to your poly lifestyle, then I agree with what a lot of people said:
See if you can assist him in getting in touch with his fears.
Also- some people are just more co-dependent than others and they have lived that all of their lives.....it's a mindset that is definately supported by our culture....songs, movies etc and that can be hard to break out of.
Compersion (the opposite of jealousy) is an amazing thing to experience, but until you do, it's hard to imagine that it's real. And, in a way, you have to just "let go" to be able to experience it. It may seem like jumping off of a cliff when you do it for the first time.....and it sounds like "letting go" is the last thing he is about to do!!