We were in a similar situation. One of our cats died so we got a younger cat to keep the other one company, they seemed to get on fine, but then the original one starting loosing the zest for life. We thought this was just being annoyed at having a younger cat bouncing around the place. Turned out he was really ill with liver failure, which we missed until it was quite late. Had to be put down last week 🙁
Although back on topic, cats can get depression and they have SSRIs for cats..
I’d take him/her to a vet to be checked out.