Okay, let's review:
A.) Your cat is 6 years old (not a senior citizen), is lethargic, not eating, gets dehydrated and is very weak.
2.) Your cat sleeps a lot and has lost 5½ pounds in approximately 6 months (assuming he is a large cat of 15 pounds, then he has lost over one-third of his body weight, which is comparable to a 150 pound person dropping down to 100 pounds).
III.) Your vet has "run all the tests", including x-rays, is administering cortisone shots, antibiotics and fluids.
Fin.) Your veterinarian's findings (after 6 months of tests?) are inconclusive.
**If I am missing some information, please enlighten me.**
What are "all the tests"?
Why is the animal on antibiotics?
Why is the animal taking cortisone shots?
My answer stands that you should seek the advice of another veterinarian if your current vet cannot determine your cat's illness.
I'm sure your vet is just wonderful, but I would absolutely question my vet if he could not find a diagnosis for my kitty, either on his/her own or by consulting with colleagues.
It seems as though your kitty is in big trouble and if he were mine, I would ask every vet in town and beyond.