What's normal for a cat to react when other cat ped away?

Yesterday we put my cat, Shorty, to sleep because she was dying. We had Shorty, and Simon. Now its just Simon. But what might cats do when they suddenly find themselves to no longer have another cat in the house? He is acting really sad, and will just start meowing. But he isn't eating. He will drink water, but he doesn't show interest in food. He ate a little yesterday, but he hasn't eaten anything today. But he is also having a hard time moving around. He just looks like he is having a hard time walking and hasn't really gotten up from the chair he is on all day. I don't think that is because of Shorty though, but I'm afraid to take him to the vet now. He just seems really lost now, but I didn't think they were ever really close or anything. I was just wondering how cats might respond to the loss of another cat? And, not that I think we will, but if we were to get a new cat, would that help at all, or would that make it worse? Thanks so much!