My cats is acting strange, possibly not feeling well?

[Candyleah]

Hi, I have a four year old fixed male cat. He has not been acting like himself - usually he is very social and playing with my other 3 cats, quite active and very healthy, proper diet, weight and all that. Since yesterday I noticed a change in his behavior, stays by himself, sleeping in unusual places, very solitary and almost like he is hiding. I am certain he is sleeping a lot more than usual, and I do not think he is eating (difficult to tell with 3 other cats, same goes with his bathroom habits, I am unsure about that.) I know he is drinking water, as I have moved him to the dish and witnessed this myself. All my other cats are healthy and behaving normally (2 fixed males 1fixed female). I am going to make an appointment to see his vet tomorrow, but the wait is usually about 2 weeks. If there are any ideas as to what could be causing him to act like this, any insight would be greatly appreciated. Thank you!