I'll reply even though I have no experience in these matters. It doesn't sound anything like poly to me which is why you probably aren't getting any responses. Not that it has to be poly or even defined at all as long as it strengthens your relationship and increases your happiness.
Your husbands experience trying to get laid is not out of the ordinary because in truth it's a whole lot easier to find a man looking for no strings attached sex than it is to find a woman interested in that. In particular on some Internet ad. He may have better luck doing it the old fashioned way and going out to a bar meeting women in real life. Or you may want to get involved in a local swingers group. I've met couples who have much better luck that way. Plus then you will be around more open minded people that could possibly help you maneuver around some of the early relationship struggles.
Also, my wife has gained a lot of weight since we got married. Sexiness us a state of mind. Sure he like every man will be initially more attracted to a fitter woman, probably a younger one too. But that doesn't mean in the long run he will find her sexier and even if he does, that sex won't have the intimacy that the two of you have. Sex without intimacy is only fun occasionally. At the end of the day the intimacy you share with him should always have him desiring you. If it doesn't then your relationship probably isn't worth it anyways.
Not much other advice I can offer really. I only wish I was more experienced.