I don't know what else to do. He has had a 4 lb. weight loss over the last couple of years. His geriatric feline profile is normal. Any suggestions would be greatly appreciated.
IMO, a cat should not need hairball remedy on a daily basis. Is he vomiting hairballs every time, or is it food or liquid with some hair mixed in? Many hairball remedies are based on petroleum products, which leach vitamins from the body (most notably, Vitamin A). For this reason, if he is getting the hairball remedy more than once or twice a week, he probably should have some kind of supplement. If, in fact, his vomiting is due to hairballs, frequent brushing, at least daily, should be helping.
Vomiting is fairly common in geriatric cats, as one or more systems start to decline. An article in "Catnip," published by the Tufts University School of Veterinary Medicine lists a cornucopia of causes, including CRF (chronic renal failure) (most common in older cats), Diabetes mellitus, hyperthyroidism (overactive thyroid gland), liver disease, electrolyte and acid/base imbalance, stomach problems (inflammation, parasites, ulcerations, tumors, esophageal hernia) and about a dozen more suspects, including pancreatitis, feline heartworm, and peritonitis.
One of my latest geriatric cats, who lived to be 19 years, was in end stage renal failure, and had been vomiting almost daily for a month or more, my vet gave him injections of Tagamet, along with a prescription of Reglan.
I urge you to take your kitty back to the vet for blood tests. At his age, things can happen pretty quickly, and though he may have tested normal awhile ago, something could have cropped up in the meantime. Also if he becomes anorexic from the vomiting, hepatic lipidosis (fatty liver disease) could result, which is potentially fatal if not treated. Please let me know how he's doing.