Has he tried doing anything about his back? Has he seen a GP about it? It could be that if he's got chronic back pain, it does mean other things are too painful (not just sex,) but my sympathies would vary according to whether he's had it looked at and tried treatments - again, not just for sex. Back pain can make many areas of life difficult.
Did he offer any other solutions? Hands, tongues, toys? FWB? How far does his no sex go? I would still want hugs and kisses and bodily contact, but I can understand some might pull back from that, too, in case it leads to expecting more.
Anyone had the right yo decide they don't want sex again, but if they're in a relationship, they should also consider what this means for their partner, especially if they do want sex, and to think about ways that this incompatibility can be managed, what boundaries there are. I know iw of a relationship where he ended up in a wheelchair after an accident - they agreed that she could get sex elsewhere, but not to discuss it at home, and that worked for them for a couple of decades or so; it probably wouldn't work for others. I know you don't want to start another relationship, but it is one possibility among others, and you need to talk about possible solutions together. If he just won't even talk about it, thst probably would be a dealbreaker for me.