Well, first off I'd recommend education - for yourself - and with him if possible - on the various flavors of non-monogamous relationships. Particularly the differences often perceived between "swinging" and "polyamory".
See if you can find out if his feelings are primarily driven from a sexual direction. I (we) are from the camp that "sex is just sex". It's not the ONLY camp. Kept in proper perspective it's potentially harmless and can add some spice to long running relationships. If that turns out to be the case you may experiment with "swinging" first to test the waters.
It sounds like your relationship itself is good - and solid (your side so far) and if in fact that is true then you two should be able to "team up" and explore some things together safely. You can limit your own personal (sexual)involvement to whatever you are comfortable with and be together with him to share his enjoyment and fantasies.
A lot of people will tell you it moved their relationship to a whole new level !
Feel free to throw out questions here. There's good people here from all possible walks of life with broad experience. More than likely whatever bridge you are thinking crossing - someone else is already on the other side