There's also the possibility your cat may have worms (tapeworm or roundworm), a fungus infection or allergies to his food (which can develop even if they've been eating the same thing for years). Again, the condition you have described makes it quite difficult to lend you a possible diagnosis as only a vet would be more qualified. Everyone may mean well here in Yahoo Answers, however anyone who suggests something may actually cause your kitty more harm rather than help.

May I suggest that you personally call your cat's vet (your parents do not have to if you are asking a question). Ask them how you could best persuade your folks to bring your kitty in to be examined by explaining your concern. If they feel the cat needs to come, maybe you could have them call your parents and discuss his well being.

I hope you will be able to get him relief soon. I know you love your kitty bunches, but please don't rely on the Yahoo Answers as much as a vets. A vet is a medical professional trained to treat injured or sick animals best.
References :
http://www.petplace.com/browse.aspx?a=2&c=264&p=1
Feline Trainer

I have lots of precancerous lesions on my skin and have had three or four skin cancers. I go to the dermatologist regularly to have my skin checked.

It does not sound like your cat has a skin cancer. They do not develop these on their bellies or underneath the protection of their fur. I don't think you have any reason to worry about that.

As to what it might be only a vet could give you an idea. I don't think it sounds serious at all.
References :

for help
http://www.fabcats.org/cancer1.html – 36k
References :

2. Be concerned if you find new growths, or if any existing growths change colors or become larger.

3. Note any areas that bleed easily or do not heal normally – these may mean trouble.

4. Groom your cat daily with a fine-tooth comb to help you notice small changes early.

5. Massage your cat often, feeling for masses or other suspicious areas. Fingers often find things that you will not see because of the hair coat.

6. Look closely at your cat's eyelids and lips and inside his mouth for irregular areas or color changes. If your cat's nose or ears are white, check them closely for scaly, bumpy or reddened areas.

7. Report any suspicious findings to your veterinarian.

Tips:
Orange tabby cats often develop smooth dark spots on their lips and eyelids as they age that are not cancerous.

Chronic infections from cat-fight wounds that will not heal may resemble skin cancer.
References :
http://www.ehow.com/how_17988_detect-skin-cancer.html