Sorry your boy has a polyneuropathy. There are worse diagnosis but it isn't something I would wish on anyone. One of mine dogs had a slowly degenerative polyneuropathy. Actually, he had a lot going on but in the long run most came back to the polyneuropathy. We did the full diagnostic set & ultimately, because my guy was only 7, I opted for the muscle & nerve biopsy. Of all the tests & all the options we tried the only thing I regret is having that biopsy done. It got us no closer to a diagnosis or possible treatment, caused my boy unnecessary pain & basically just added to the nerve damage making him worse. In a sense, he only had one nerve left & darned if we didn't get on it.
I have a few other suggestions that you may want to research. Yes, definitely get the full thyroid panel & parathyroid. Plus, ask for an ACTH stim test to check for Addison's or Cushing's. Don't just do acetyl l-carnitine, add CoQ10 & consider alpha lipoic acid &/or grapeseed extract. A b-complex tablet if you're not already giving one is a very good idea. Truly, seriously, really, really, consider B-12 injections. B-12 sub-q is fine. No need for painful IM injections. They are easy to give & of all the things that was one of the most helpful.
Without knowing causes you don't have any idea of rate of progression. My guys first symptoms, oddly urinary & swallowing, started before I adopted him at age 3 yo. His mobility was first impacted when he was 7 yo. We finally had to let him go just before he was 10 yo, 3 yrs after diagnosis. Way too soon but... Well, at least we had a lot of good time together even after diagnosis, including his first trip to the beach. He required help to stand up but could still walk on his own up to the end. No miracles to offer you but there is lots of hope for more time to enjoy sunbeams & more.