Is he drinking and urinating more, less or the same as previously?
There are a number of problems that cause weight loss and lack of appetite. Most are not good. Kidney, liver and heart disease are high on the list as are some infectious diseases. Regretfully, diagnosis requires blood tests and/or other diagnostic tests. Treatment is based on diagnosis.
Recheck the teeth. Look for calculus on the tooth. This often hides lesions in the enamel on the neck of the tooth and sometimes abscesses. Most of the time, these do not cause weight loss or lack of appetite.
Do you have access to human doctors and/or drugs (antibiotics)?
Let me know if you have follow up questions.
Both kidney and liver problems usually have increased water intake and urination.
If possible, have one of the docs listen to the cats heart and lungs. Murmurs in cats often indicate heart disease. The normal rate is around 170 beats per minute. This is hard to count until you get used to it and learn to count fast. Over 200 may indicate heart disease.
I've had some luck treating non specific illness empirically with antibiotics. Pediatric amoxicillin (50 mg twice a day) or augmentin (62.5 mg twice a day are good choices if available.
I've used prednisone empirically also....back in the days before easy access to blood work, ultrasounds and other instruments of modern technology. In addition to being a good antiinflammatory, it also stimulates appetite. I'd use antibiotics first and 5 mg of pred a day for no more than 5 doses if the antibiotics don't work.
Another thought is parasites. Roundworm and tapeworm are the most common in the US. I do not know what others might be common in Yemen. Let me know if you or your docs know. Pyrantel is a good treatment for roundworm and praziquantel for tapeworm.
There are some veterinarians, particularly working in agriculture, in Yemen. Are you able to find any to ask about common cat diseases and parasites that may not be on my radar as an American vet?