gogosean, cats do get lonely as Emland and Tigerfeather have stated above. They require the mental and physical stimulation that socialization brings. I am sorry your cat is going through this problem. There are some things you can do to help him. Is it possible for you to buy or borrow an enclosed "cat condominimum?" I am referring to the very large cages that you can put in a play area that will challenge him while not completely cutting him off from others. The other animals can see him and stick a little of their paw in without actually coming into contact with the skin condition. The best part is that he can see them and won't feel so isolated. They can still interact and entertain each other a bit without the fear of contact. If it is possible, you could borrow a very large dog crate, and place one of those cat "playgrounds" inside of it. Place and hide toys around inside of the "condo" that will provide stimulation for him. When you are home, use those cat toys that are on a long stick with a fake mouse (or some toy that dangles down), so you can poke it through the cage and play with him, if he is suffering from any skin condition in which you yourself cannot pet him and play with him for fear of getting infected yourself (ringworm, sarcoptic mange,. ). The cage wires will create an added stimulation for him in that he can't get to the toy so easily. He would enjoy himself immensely with that and it would be the kindest thing to do for him while he is recuperating. Here are some sites that will give you an idea of what I am talking about, in case I am not making myself very clear.
Pussicat Playhouse with Tunnel Cat Gym - CatsPlay.com - Fun furniture, condos and climbing gyms for cats and kittens.
Cat furniture - one of the best cat trees, cat condos and cat furniture
These can run on the expensive side but these are just examples. I don't know what your financial situation is but you can do a bit more research on your own and see if there is anything out there that might fit the bill that is less expensive. If you are at all handy, you can pretty much build one yourself with bits of old carpet and wood. Check your newspaper's classified section and see what tag sales are in your area this weekend, and see if anyone is selling an old large dog crate (the kind that you can fully see into that is made of steel). You just need to make sure that it is a crate large enough to put in a little "kitty condo", a litter box, and toys. It actually will be something that they all can use when your cat is completely healthy, by just leaving the door to the crate open all the time. It can also be used again in the future, in case you have another problem in which a cat needs to be isolated.