What's wrong with mouse?

Hello all, I hope this is the right forum. I have what appears to be a sick 2 1/2month old kitten on my hands. A friend of mine dropped him off from her very overpopulated (kitty and dog) home. I have had him since he was 6 weeks and he was like any normal kitten untill today. Tonight more specifically. I have a 2 year old in the house so he was made to be a cat who plays outside in the day ( once he was old enough) and comes in at night to limit the kitty box use. He comes in tonight and he whines at me pretty constant, he will walk but not far before he plops down like hes out of energy. He even let me roll him on his back and rub his belly which would normally be an attack on me. He hasn't had his shots yet as I was waiting for next month when he would be netured. He is not purring and is uninterested in food. He doesn't seem to be hurt in any way physically no thorns or anything in the paw. Just sick. He was also trembling some but not like a seizure. He doesn't seem like he will last the night and he was bouncing around earlier. If he does last the night i'm hoping for some insight on what this might be. If he doesnt last the night do I have to worry about my almost 3 year old cat who isnt updated on his shots?

I wish I could help you more but I would have to say a vet visit is in order, NOW. Take your other cat in and get it up to date on shots, espesialy if you intend to bring a new one into the home. Also take the new young one in to get looked at. You never know what could have happened and only a vet can help you. You say it came from an over populated house, a wet check is mandatory. Any disease could be presant and you won't know untill you get it looked at.

I'm not sure I understood your post correctly but it seems one of the cats has been forced to be more of an outdoor cat. If this sickly cat is the one that's been outside without neutering or vaccinations, it could be anything. Unfortunately this cat will also have already brought in whatever he has and could well have passed it on to the other cat if the problem/illness is catching.

Unfortunately putting a cat outside without neutering or vaccinations is like playing Russian roulette - you take your chances.

I echo the advice of others - take the cat to a vet. Unfortunately it may cost you more than the vaccinations would have cost you and possibly have prevented this if it's something that would have been preventable.