My 13 year old cat is showing signs of bad dental health, I need help.

Question
My cat Khepera, one of three in my household is not doing so well. By my research, it seems to me he has developed some sort of dental disease. He is drooling when I go to pet him, or when he awakes from a nap. He used to be an overweight cat, never would I think he would be so skinny to see his hip bones stick out. I've watched him eat, and he chows down his meals. He's had bad teeth his whole life, and they've always been a bit sensitive. I know by my research, he's symptoms could mean a lot of things. I was hoping that you could lead me a step further in trying to find out whats going on. I'm very concerned and I hate seeing this cat so thin! My family doesn't have the money right now to take him to the vet, but if its something serious, than were gonna do all we can. Thank you very much for your time and consideration. -Kara

Answer
Bad teeth can cause poor digestion because the food is not getting chewed up properly.  In addition, there is probably an infection in the gums present.  In older cats that are losing as much weight as your is, I am always worried about kidney disease and high thyroid hormone levels from a thyroid tumor.  Both of these condition are very common in old cats. Simple blood tests may tell you.