What's wrong with my kitten? :(?

My cat, Switch, is about 3 months old now, and at 1 month old, he had an upper respiratory infection. He had a swollen eye, sneezing, loss of appetite for a couple days, and very bad congestion. He seemed weak only when his congestion was really bad, and wasn't eating. The vet gave us antibiotics and eye cream for the swelling, and he was back to normal within about a week and a half. We just got a new cat, Blade, who is 4 months old, and has been here for a little over a week. She is completely healthy, had her shots, and the cats have been playing together all week. The past few days, Switch has been abnormally very mellow, and at first I thought he had just pooped himself out from playing so much. But now I'm looking at him, and his eyes look kind of puffy (possibly from sleeping so much?), and he is not sneezing at all. He is congested, not as bad as before, but has now lost his appetite again. I took some soft cat food and forced it to his mouth, and he does eat a little bit, but nothing like he normally does! When he does walk, he walks slow, and just jumps on his bed and sleeps. He just seems so weak. He also is breathing very short, shallow breaths :(. He has not had his shots yet. I'm very worried about him! Please help!