Your b/f sounds mono. I totally agree with Redpepper and I experience that with my poly partner. He loves someone else as well as me and yet I feel more loved than I ever have in my life. Having said that I still get your b/f's words flashing into my head from time to time. They don't come up from any lack in reality but from my interior sense of lack.
The abundance of love is a great concept but what also helps me is thinking about things that are important to me apart from my partner. I love him intensely but I still need other things in my life to make me happy. When I was in a unhappy marriage I didn't think that would be the case. I thought that if I had the love of my life, nothing else would matter, but it does. Our love isn't enough to replace my need to do work I enjoy and is sustaining, or my need for other sustaining and enjoyable friendships. It helps me to put his need for his other love into this context.