my cat is dizzy and losing balance, please help

[lovekitties]

Hi all,
One of my cats is showing symptoms which are similar to some symptoms posted in other threads, yet I can't find any obvious correlations.

Our vet is stumped and cannot suggest anything constructive yet. We're considering doing a test for Thyroid soon to check for that too.

He is a European shorthaired male [adopted stray], neutered.

SYMPTOMS AND INFO:
My cat has been weak on the hind legs, walking as though his is dizzy. He loses his balance easily, and especially when he shakes his head - he just falls over.
He is lethargic most of the time and barely has the strength to walk around. He eats reluctantly most of the time and i have to hand feed him to entice him to eat. Unless it is something he really feels like and purrs when i offer it to him.
He often kind of 'stares' very often with a dead gaze...
He moans when i pick him up. He does enjoy being brushed... I suspect he doesn't have the energy to groom himself. He seems to have dandruff too now.

He has just started shaking a bit today... hind legs and also his head. Its not constant though, i just noticed it for a while this morning.
I cannot comment about his litter usage as I have 3 cats.

His bloodwork is all fine, no sign of diabetes either. Thyroid hasn't been tested yet.
He eats wet food, mostly canned Whiskas as he doesn't like much else. Sometimes he accepts to eat the raw food that I feed my other cats. Now he is eating canned tuna sometimes and ham.
He has lost a substantial amount of weight too.

We can't determine his age as he was adopted as an older cat. He is definitely older than 5yrs. He does go out, but not very far, mostly in surrounding gardens or small field next door.

We vaccinated him a few months ago and recently had to sedate him to take his blood to be tested. The sedation had a very strong impact on him - it knocked him out very much. His symptoms started before the vaccination.
He had a bad ear mite infection a short while ago, and we administered canaural for just over 3 weeks. It seems to have got worse around the time all this was happening.

He had one good day last week - more energy and he even jumped onto a low wall! That was the only day and after that, back to usual.

We are in constant contact with out vet who is also baffled by this.

Any suggestions would be appreciated!

Thank you!