I think if he knew how you felt he wouldn't want you to pretend for his sake. This isn't good for either of you. I would much much MUCH rather a partner tell me they don't want me like that anymore than to "sacrifice" their body to me when it makes them feel bad. It would seem this isn't going away, and if he's really as connected to you as you say, sooner or later he will realize how you've been feeling. Better to tell him honestly and see what the relationship will change into.
Hopefully you two can retain a close and important connection even if you're not sexual partners. But don't lie to him any more by pretending to be interested when you're not, sooner or later it will erode the other good things about your relationship.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.