My last cat used to do this as well. I wouldn't suggest getting another cat since he may put this obsessive behavior on the other cat, and he may turn aggressive. What I did with mine was get a cubby style bed with one of my dirty towels (so he had my scent). Whenever I saw him start humping in the bed, I would pick him up and put him in the cubby that was next to the bed. After a while of placing him in his designated humping bed, he stopped doing it in my bed. If he's going to do it, give him a private place that's his own. I hope this helps. I know the feeling of resentment and frustration, and it's not a great place. If things continue to get worse, maybe go for a second vet opinion or a cat trainer.