I'd say there are two separate issues going on here.
1. I'm not sure polyamory is really what you need in this situation. To be a bit blunt, you need a good dicking. : P You're focusing on this hot neighbor because he seems accessible, but if your husband gave you to the go-ahead to explore your options a bit, you could go on OKCupid or something like it and find a guy with a vigorous sex style on about 10 seconds flat. This doesn't mean you need to find an extra boyfriend or fall in love with this other person; sex alone should be enough.
2. Even if you manage to sort out the sexual issues in your relationship (either by getting your husband's libido up or finding others to bone), you may still wish to work on the root causes of the ED. While it's great to get sex to avoid overeating, it would still be nice if you weren't at all at risk of plunging back into the spiral should your sex life take a dip.