Why does my cat seem sad and uninterested in anything?

He recently went to the vet to be treated for an upper respiratory infection and got off medication the 13th of this month. so this is the fourth day off treatment. does that have anything to do with it? we got a new kitten 2 days ago and he seemed very excited and happy and he tried playing with her but the new kitten was scared and wouldn't play. and all he did yesterday was sleep and he barely even ate. we tried playing with him but he didn't care. so we thought maybe he needed more rest and now this morning he's the same. what could cause this? do you think he's sick?