I had a cat who not only wouldn't put up with another cat--she wasn't too happy about having another human living with me, either. (As for the other human, turns out Keke had better sense than I did, LOL!) She spent most of her life as a very happy only cat.
You obviously tried your best with Oliver, and it's certainly not your fault that you weren't warned about his only-cat inclination. I'm sure it's hard to give him up, but he'll be happier where he can be sole lord and master.