I’ve had that happen before, only it was my dog that died and my cat was very depressed for a long time. All the cat’s i’ve had were very much like people, and your poor thing must be grieving big time. I wouldn’t get him a new friend (at least not right away). Be affectionate and overtly kind to him (play with him a lot). Just as with people, we need time to mourn but we can’t get caught up in our pain. If a friend dies, it’s not like tomorrow we will go and find a new friend. Give him/her some time, and if he doesn’t perk up in a few weeks to a month, I’d get him a new friend. But be careful with that, because some cats who lose a close friend become very aggressive to other cats.