Cat stopped eating

A few days ago I brought in a stray cat. He is one of 5 cats I have been feeding for approximately 5 months. He usually stays around my house but sometimes goes to other houses. He has become very friendly so I decided to bring him in for the cold weather. Keep in mind this cat has a very good appetite and is very active. Looks very healthy too. I got him vaccinated and fixed this past year, along with the other cats. When I brought him in I put him in a separate room because my cat does not like him. Anyways, he had been doing good, was eating his food as usual, playful etc. On the fourth day after work I noticed he looked a little tired and was just resting in bed. He didn't greet me or anything. I did not see any vomit or diarrhea. His litter box had normal bowel movements and he had been urinating. This was around 6pm or so, so I assumed he ate and drank. He has always had free access to kibbles and water. Today he remained in bed most of the day. Maybe got off like once or twice. He used his litter box once to urinate. I don't think he ate any food or drank water today. I gave him a few syringes of water. I offered chicken, fish, wet cat food but he wasn't interested. He just had that unwell feeling. Kinda sleepy. In the evening I usually give him wet food and a few Friskies treats. That's a daily routine. What can be the issue? Any advice is helpful.