This is exactly the situation my wife and I are in. She has a long distance relationship with her poly boyfriend that recently became intimate. I have made some friends, and there is one woman inparticular that I know I could develop feelings for, but for now, I am still monogamous. Looking for something, sometimes makes it that much more difficult to find, so I guess my advice to him is, don't look so hard. It is not a race and not a situation where fairness dictates that if you have a boyfriend, he should have to have a girlfriend. All things in time.
I understand his frustration, I have experienced it too. It is difficult to find a woman who is happy and satisfied in a secondary role, I think men take to that easier (not to over-generalize or stereotype, that has just been my experience). I recently had an epiphany when I realized that a lot of the uncomfortable feelings I had been experiencing were tied to me being jealous of her relationship. Not that she is in love with another man, but that she has him, she has me, and I only have her. My fear was telling me that I am unworthy and that I would never find anyone else that loves me too. An irrational fear once it faces the light of day. I love my wife and our relationship is awesome. If our relationship is the only one I have for the rest of my life, I should consider myself very lucky to have it. I really don't need anything else. If something else develops, fantastic, but if not, oh well. Try to be mindful of this type of jealousy, your husband may experience it and not even know where his insecurity is coming from. Now that I realize where my unease has been coming from, it is much easier to be happy for my wife and the love that she has found. Patience I believe is the key. If your happy, he should be happy for you.
Good luck,
Skater21