Hello, my name is Dr. Kara and I have over 20 years of experience as a veterinarian.
I am sorry to hear that Basil's coat isn't as nice as it once was and he is losing more hair then usual. I am also concerned that he has lost so much weight that he is feeling bony.
Because the coat and skin are less important then internal organs like the brain, kidneys and liver they tend to suffer first early on in a disease process. They may no longer get the nourishment that they once did if a cat isn't at their peak of health.
It sounds like simple things such as worming and flea control have been tried.
If only Frontline has been given though that is only good for flea and tick prevention, it is not a wormer.
At this point we need to look for underlying internal disease processes, such as liver disease, immunosuppressive viruses like feline leukemia or feline immunodeficiency virus, or feline infectious peritonitis, inflammatory bowel disease, or even infiltrative intestinal cancers like lymphoma.
Ideally he would see his veterinarian for another examination and blood tests to make sure that he is still healthy. I would start with a complet blood count, biochemistry profile and tests for feline leukemia and feline immunodeficiency virus. Other tests will be based upon the results of those.
In the meantime it may help to add an omega 3 fatty acid supplement. These are terrific for supporting skin and coat health. Good veterinary products are Derm Caps or 3V Caps. If you choose to use a human product or a pet store brand be aware that these are nutraceuticals which are not regulated by FDA and as such there are some very poor products with little to none of the listed ingredients present in them. The recommended dose is based on the EPA (eicosapentanoic acid) portion of the omega 3's. Give 10mg to 20mg of EPA per pound of body weight once daily. The supplement should have the amounts of each omega 3 listed per dose and if they aren't listed then the supplement isn't likely a good one.
It may also help to feed a canned food rather then a dry because it is easier to chew and swallow, if he has sore gums and teeth, and it will increase his fluid intake and many cats don't drink enough water. Make sure to do any diet change gradually or you may end up with an upset stomach and diarrhea.
Please let me know if you have any further questions.