Older cat losing weight

Question
My beloved Beanie, now age 16 has lost quite a bit of weight lately (13 lbs. to 8 lbs).  He once was a large, heavy cat, but over the last six or eight months has grown very thin, despite the same diet and exercise.  I have given him little bits of milk and even very small portions of table food from time to time in an effort to help him gain some weight, all to no avail. He has no other illnesses or impairments that I am aware of.

I hope that his weight loss is not a precursor to the inevitable.  It would kill me to lose him as he has been part of my family since he was a kitten, but I am well aware that pets do not live forever.  Our female cat and Beanie's litter mate, Sissy, still seems to be thriving.  She has lost a little weight, but it is not nearly as drastic as Beanie's weight loss.

My question is this... Is there anything I can give Beanie to help him gain and retain some weight?  I don't know how much longer he will live, so I want his twilight years to be his best.

Thank you very much!

Answer
Hi Renee!  Although it is considered as normal for an older cat to lose some weight, this has been a very major weight decrease in very little time.  The weight loss would have to be caused by something.  You may want ot take Beanie to your veterinarian.  Older cats tend to develop Hyperthyroidism, which would cause severe weight loss.  Hyperthyroidism is diagnosed with a simple blood test (called a T-4 blood pannel) and is successfully treated with medication.
Weightloss can also be associated with more severe ailments and diseases like renal disease or even cancer.  Again, blood work would also diagnose these ailments or the like. The first thing that would be at the top of my list for possible ailments would be the Hyperthyroidism.  Even pets with cancer or renal disease can live happily with a few special accomadations like sporatic subcutaneous fluids (fluids given under the skin), steroids, vitamins or supplements...so the diagnosis of any of these things is not necessarily a death sentence, as veterinary medicine has come a long way!  
To gain weight, you may want to talk to your veterinarian about Science Diet Prescription Diet A/D.  This is an extreme calorie and fat induced food that also stimulates appetite.  It comes in canned/moist food and dry food and will pack on some weight.  NutriCal vitamin and calorie supplement is also a wonderful way to pack some weight on.  This is almost like a gel that is in a tube.  It can either go onto Beanie's food or you can simply rub some on his mouth or gums.  NurtiCal is very inexpensive and I believe it is sold at most pet stores as well.  Innova brand cat food is also very high in fat and calories and can be purchased at any pet store.  
Getting some weight on Beanie definitely would be a good idea, as it will make him stronger to fight whatever is ailing him, and make him much more comfortable.
I recently lost my old Bailee girl from cancer.  She was 18 and had renal disease and hyperthyroidism since she was 10 years of age.  She was diagnosed with cancer when she was 15 years old and lived 3 happy years with me.  She also began losing weight very quickly and the above remedies helped her gain weight.  I hope that Beanie is helped by this as well.
Please keep me posted and give Beanie and Sissy tons of kisses for me!
Hilary