It sounds like you suspect the back problems are not the "real" reason, because if he valued your sex life, he'd prioritise seeking help for his back and exploring ways to work around his back problems. If you love someone then you want them to be happy, generally, and surely you would hope he'd at least want to explore options with you.
It's fairly common for men to lose libido in middle age even without there being any physical problem, or excessive porn use, or an affair. A friend saw a sex therapist about loss of male libido, and apparently a substantial portion of the clients were men in this situation. In some cases it's the result of other problems in the relationship, or loss of attraction, and in those cases both partners might feel quite differently in a new relationship.
You might find the Dead Bedrooms board on Reddit helpful. There are many people on there who can empathise and some have managed to revive a sexless relationship. If you search mumsnet for "sexless relationship" you'll find plenty of threads, mostly by women whose partners don't want sex.
It sounds like your DH is ready for slippers and companionship, but you're full of life and want to enjoy a full relationship with him. It's such a shame. I hope you don't settle for this; you're a long time dead. I had long periods of feeling unwanted by my ExH and I look back now at those long, lonely nights, feeling that my love was wasted on a man who didn't want all of me. It took a long time to end things properly. I'm now 50-ish and enjoying a relationship with someone who I have such joyous fun with, and I wish I had called time on my marriage sooner.