This is one well-known sign of arthritis. And chiropractic doesn't work for that.when he is just getting up from laying down for a while he seems really weak and even falls over in back.
Also, it has nothing to do with the thyroid.
Two links for quick but important information.
http://cats.about.com/cs/healthissues/p/arthritis.htm
http://wwworigin.weather.com/outlook...s/articles/c33
Much more if you go to Google and type in "arthritis in cats"
And once again I urge you to go to Google and type in these search words
cat has weak hind legs
sacral/lumbar problems in cats
There is a tremendous amount of information there that you absolutely have to read.
As for vet care, I urge you to try again and do everything you can to find a competent, caring vet for your baby.
In addition, it would be wonderful - the best thing, not to mention very necessary - if you could take him to a board-certified veterinary orthopedic specialist for a thorough exam and advice.
At this point you still don't know what's going on with him and you need to find out in order to be able to do your best for him.