Cat lethargic lately

I'm concerned about my eldest cats health. He's under two so not old at all. We have a new kitten who he loves playing mother too and wrestling with the new LO. The past two days though he's seemed distant has been sleeping more than normal not eating or drinking normal either. Just lays on the bed all day. When the kitten comes around to try and play the eldest cat hisses at him which he's never done before. I'm concerned something is wrong. Should I take him to the vet?

I would because of the change in eating and drinking habits.