I wanted to leave a brief post, just about what my experience was when in a similar situation.
When the guy I'd been seeing as a secondary partner (and considering having move in with my husband and me) kept telling me that poly was fine, and that he was OK with what I did, and of course he would date other people, but showed no signs of doing so and seemed to want to spend all free time with me...
I just went out on a couple of dates. His reaction, and my observation of how hard he was struggling was enough for me to end things with him. We were good friends so managed to keep our friendship.
Though I know it is bad form to make decisions FOR people, I needed to see he could cope with my dating, and find other relationships so I knew he was getting his needs met - I ended things. This was after a lot of attempts at communicating, and efforts to ensure he knew how important he was to me. 5 years later he admits that he just wasn't cut out for poly, though he may have been open to a mono-poly relationship. That it just would've been stressful for me, and him. Is your boyfriend interested in dating others now himself?
Nycindie has great points, I also was not jealous so much of my husband dating others, but when my secondary partner started dating new people, I found that much harder, especially as it was somewhat of a LDR and I wasn't seeing things firsthand, and I didn't feel like my bf's and my relationship was well enough established to feel secure that it was an IMPORTANT relationship to him, that wouldn't just end the moment he found somebody closer/more in common with him/better than me.