To be honest, you can't. Your best bet would be to table the whole thing until you have given him the courage to see that love and sex are not the same thing. Really what you are looking for is a situation where a new love interest adds something to your relationship that is above and beyond the basic necessities. Work on the relationship you already have without pining for something that may or may not be a good addition to your life.
If you feel something is lacking in your current set up then work on the current set up to get what you need out of it. What do you think you will get out of a poly lifestyle that you aren't getting in your monogamous lifestyle? There are ways to add those elements into your relationship while building your partner's trust and confidence.
It's never going to be easy to introduce polyamory to a closed relationship, it can be done and done successfully but it isn't an easy or fast process. Check out some of the other forum posts on this subject here at EF. There's world's of good advice and suggestions from those of us deep in the trenches!