There is a supplement called Nutri-Cal that is sold in pet catalogues, pet supple stores, pet stores and vet's offices often which is a high calorie gel type of substance many cats love, and lick off your finger like a treat. This is generally utilized for cats who aren't great eaters or have trouble digesting their food appropriately.
However, I would first make sure there is not another problem going on. Fleas can cause anemia which in turn can cause weakness and less of an appetite, but more commonly, a flea gets ingested when grooming or biting due to the itchiness and this causes tapeworms, an intestinal parasite which feeds off the lining of the intestines and this could be one reason your kitty still eats but is losing weight. I would definitely drop off a stool sample with your vet to check for other parasites, treat for fleas with advantage or frontline, and let the vet know you have seen fleas and worry about tapeworms. A cheap, one-time pill called drontal (or droncit) can be given to get rid of this problem.
Another thing to consider in an older cat is hyperthyroidism, which often first presents as a good appetite along with weight loss. This is because hyperthyroidism is caused by a (usually) benign mass located on the thyroid gland, which causes an overactive metabolism. Over time without treatment, the cat starts to "waste away", and other signs are noted as well: hair loss, more vocalizing, weakness, vomiting, etc.
Hyperthyroidism has a few options for treatment/management.
There a 3 main ones:
1. Lifetime management with daily medication. The medication isn't expensive but after years it does add up. With your cat being 17, this is the most feasible choice.
2. Radioactive iodine treatment. This is a one time subcutaneous injection (under the skin) easily given, but does need to be done at a specialized facility, which may be far from you, the cat needs to board for a few days afterwards to clear any iodine safely through the urine so you aren't exposed yourself, and treatment can be expensive.
3. Surgery to remove the mass, expensive, but a cure nonetheless.
Hyperthyroidism can be diagnosed easily with a blood test from your vet.
Start with treating the fleas and possibility of tapeworms and possibly try the Nutri-Cal at this time to get weight on faster. If not seeing improvement, you may want to consider having the blood work done.
Best of luck, and let me know if I can help further.