Sex drives change over time. My husband and I have been together for 17 years and there’s an ebb and flow to things. I’ve been in the no libido position and so has my husband. We’ve also had health issues through the years that prevented us from having sex. However, it never stopped us from being creative to get each other off. Yes, it’s hard to get into it when you’re not feeling frisky but that’s part of life - doing shit you don’t want to do. It may also mean that you jerk off more. There’s nothing wrong with you jerking off while he’s next to you stroking or kissing you or stimulating you in some way that turns you on. Maybe you agree that getting a massage with a happy ending is agreeable. That’s one way we’ve dealt with our own situation.
The real question you’re facing is about an open relationship. It doesn’t sound like he’s into that solution. Bringing someone into your bedroom is a difficult thing. I did it with a previous partner; it is filled with pitfalls. If your partner has no libido with you I’m not sure how a third party helps? If he doesn’t want to have sex, how does another person help? Is he going to sit in the corner and watch another man satisfy you? If so, I can see his reluctance.
Adding someone to your bedroom ignores the problem.
If his situation is permanent and unlikely to change then you have to decide if you can compromise on the amount of sex you’re having. Or find some equitable compromise.
These are just opinions from an older guy who has been there and done that. Good luck.