First of all, it's just so great that you are caring for him. Poor little guy, I can't imagine how people do the things to animals that they do. If nothing else, he knows he's cared for and loved and that alone is a huge thing. Because of you he didn't die alone, scared and in pain.
What you are describing is very likely feline herpes virus. He might also have damaged larynx (or not, some cats never meow, but from what you're telling me, it does seem very possible).
So, in the meantime, you are already doing what you should be doing. A lot of the danger with these ongoing viruses is with other illnesses that he might pick up and then the additional virus is a complication. At that point, he might have some issues. So you want to keep as good care of him as you can, keep him as healthy and happy as you can - like I said, what you are already doing. He could live a long life, and, however long his life is, he's spending it happy and loved - thanks to you.
I take it that he eats well, since he's gaining weight. If you are feeding him only dry food, than you have to be aware of his hydration levels, make sure he gets as much water as you can get him to drink. Dehydration is very dangerous, especially for a cat with health issues and cats get most of their liquid from their food. Canned food provides a lot of hydration but dry food doesn't. If you can't afford canned food and he doesn't seem to drink a lot of water, you could try getting him a jar of meat flavored baby food whenever you can. Cats love it and it has tons of water in it.
I recommend not giving him baths. Getting him wet could be dangerous, cats don't do well with that at the best of times and you don't want him catching a cold. There's no reason to bathe him anyway. If he gets into a mess, just wipe it off with a damp cloth and flea baths don't work. Even flea powder is better, although really, unfortunately, the only flea stuff that works are the expensive ones like frontline.
Btw. feline herpes doesn't spread to people, only other cats, so that isn't something to worry about (some people have asked about it, so I thought I'd let you know !).
If at any point, you can save up to take him to the vet, then there will probably be some meds he can take for the symptoms. I could be wrong about the feline herpes but whatever it is, it is a chronic virus and the advice would be the same as above.
Just hang in there :)
Give him a hug for me and please let me know if you have any other questions or concerns about him, ok?
Thank you very much, its comforting to know im doing all i can for the little guy and ill be sure to pass on the love ^_^