Walking flat footed on hind legs!

My 51/2 yr old Maine Coon is walking flat footed on his hind legs and unable to jump. This started a week ago. Now, he can still walk but his legs are starting to go out from under him, like he has no control of them. he was completely healthy prior to last week. Now, he's constipated (no poop in 5 days) and I suspect it's all something neurological. He saw the vet on Monday: she did a quick test for diabetes, it was negative. She did an X-ray and nothing but an area near his tail that could have possibly been the start of an abscess which she gave an antibiotic injection for and sent me home with three days of antiinflamatory pain meds.
Thursday: no better, back to vet for a recheck. Checked fluid in suspicious area near tail and nothing there. Gave enema for constipation and sent home with stool softener. Also referred to neurologist. Neurologists at ER vet had no clue whit it could be other than likely neurological. Suspected diabetes too but the quickie test at the other vet was normal. Anyone have any idea where to go from here?
He doesn't seem to be in any pain, still eating, drinking and peeing. Very flat footed and sudden onset. Nothing seems to be broken.
He's a handsome, strictly indoor kitty with no other cats in the home and no previous health issues.

I signed up to this forum tonight in desperation for any input this community may have.

Thank you in advance.

Poor baby. This site mentions hypokalemia (low potassium levels) as a possibility, plus a few other things. you get to the root of the problem.

The quick blood glucose test, similar to  how a Human diabetic pricks is or her finger to get a adrop of blood and applies the blood to a test strip and puts it into a meter and gets a reading a second or two later, only tells what the blood glucose level is at that very moment. To get a better sense of blood glucose levels, the frucostamine needs to be done. If a full blood panel work up was not done by the regular vet or the ER, bring your cat back to the vet and insist on it and make sure the fructosamine is included.

Walking on the hocks could be neuropathy Some diabetic cats have this but it can be caused by other medical issues.

My Sandra started having a hard time walking very very similar to what you are describing back around December/January.  After testing, they found her kidney disease had progressed.  This caused her potassium to go dow to 2.8 mEq/L.  The normal range is 3.5-5.1.  She was walking flat footed on her back legs and stumbling around.  They put her on a potassium supplement which brought her levels up to normal.  It helped a little, but she was still weak in her back legs.  They also found her phosphorus level from the kidney disease was very high, at 15 mg/dl (the normal level should be I believe below 6).  The elevated phosphorus increased her calcium.  She's now on a prescription diet for renal failure as well as a phosphorus binder.  Ever since she started taking the phosphorus binder, her walking has gotten better.  She also has subQ fluid therapy every other day.  She still a bit slow, but not stumbling and walking flat footed like before.  But she also has advanced arthritis in her spine and for this she is on Adequan.  She is stable right now.

I think first you should have your vet test the potassium levels.  Here is a helpful article about low potassium in cats:  http://www.felinecrf.org/potassium.htm

Your cat is still young, but maybe your vet can also do a kidney panel as well if things don't improve.  

Sending prayers that your baby gets better.    

I forgot to mention, Sandra is also on Miralax for her constipation.  Kidney issues can cause constipation in cats.  Hopefully your cat's problem turns out to be a simple matter and not related to kidney disease.  

I was going to ask if there is arthritis In the back joints? I think the above posts ^^ said it well about the various minerals.

What did the blood work say? Did they do a CBC (complete blood count) and Chem (minerals/organ functions) profile?