Teeth Whitening

Now we have an outdoor cat, and he’s at least 10 years old. He was a stray, and we adopted him. He was very healthy, and we take good care of him, but now that he’s getting older, he gets ill more often. Last summer he almost died from a bad infection, he couldn’t eat and he refused to drink so he was EXTREMELY dehydrated. At our veterinary place, they said it might be something more serious, so they told us to do a blood test, but it was way too expensive, plus all the other things like the medicine and the check up would add up to a lot of money. My parents let us keep cats, but they aren’t willing to spend a lot of money on them at the doctor’s place, so my sis and I had to use the money we had. (We don’t have jobs right now.) The doctor said that it might just be an infection, or it could be something a lot worse, so he advised the blood test. We couldn’t afford it, so we hoped for the best and simply bought the antibiotic against infections. We kept him in a little cabin house, treated him with antibiotics, and literally stuffed a pipet into his mouth with water and milk, and made him eat. Soon he started eating himself, and completely recovered. We were so glad because the doctors hinted that we may have to put him down, and we had a hard time settling with that.
This year, he was fine until a few months ago he developed a big bump on his spine by the tail. He rarely goes off into other property, and doesn’t search for food, and he is neutered. But we have other strays wander around our house, so he gets into fights. I thought that’s what it might be, so we waited it off. After several weeks, that bump opened, and puss was literally flowing from it. It was actually dripping down like a fountain from his back. We cannot afford another doctor’s visit, I know my parents won’t do it, and I don’t have a job. So we took some natural remedies to apply to the wound, made sure it was “animal safe” and cleaned the wound. In a week or two it healed, but next to it, more began to appear. They’re like sores. Then the skin turned black around the wounds. Right now, his fur is growing back on that spot, and it looks fine.
But just last week he developed a bump again, but this time on his head by the ear. It wasn’t as big, maybe the size of a blueberry, but he always scratched it and now it popped and turned into another big scab. I don’t know what’s going on.
When he used to hunt, even a few years ago, he was much healthier. Now that we feed him dry food, very quickly did his fur turn matted and his teeth got yellow. The first time we brought him in to the doctor’s, he was over 5 years old for sure, and the doc thought he was 3 yrs old because how white his teeth were. This was because he hunted ant ate “natural food.” Now, he started doing a lot worse, I think that dry food might be a factor. We buy supposedly good brands that promise “nutrients and vitamins” for a healthy cat, but I certainly noticed the difference when he started eating it, and it wasn’t good. We try different brands, and it’s always the same. He was already old, when we adopted him, because we lived in our house for a while and saw him from a distance for several years before he got used to us. But he was so healthy, now that we began feeding him dry food, his health began to go down.
I don’t know what to do… Maybe we should change the diet, but we can’t afford expensive foods, and I don’t want him wandering around hunting either because he’s getting really old.
Anyway, if anyone reading this has any ideas what might be the problem, please inform me. He’s not outright suffering right now, he still plays and shows affection, but he has nasty wounds appearing, and I fear it may get worse. He doesn’t much pay attention to them, but he has been much less active than usual and I fear they may get worse. Now again, we can’t afford a veterinary, I’d be more than happy to take him, but I just don’t have a job right now… I know it won’t be just a check up, it will be a blood test a something else test la da da da da, and in the end, there will be a huge bill for like 200 bucks.
So if anyone has experience and this is a familiar case to them, please answer and tell me what this could be and what could be done…
Thank you!
Here are some pictures I took. These ones in the picture already healed, but now he has a wound on the head. I just want you to get an idea of what it looks like:

http://picasaweb.google.com/EricaN83/Tommy#5348063296318683922

P.S. I know it’s messy–it aint our house, it’s our garage that we’re remodeling:

http://picasaweb.google.com/EricaN83/Tommy#5348063854899484658