I am definitely not an expert, but I have kept an elderly dragon that lived to be almost 13.
Although Gizmo lived a relatively normal life right up until the moment he passed away, I'm inclined to think that based on the sudden behavior and neurological symptoms, maybe your old guy suffered some kind of stroke?
Its really up to you whether you want to take him to the vet, but to be honest, he's really old. If he were mine, I think I'd just try to keep him safe and comfortable, and spoil the heck out of him until its time.
Take out all the things in his cage that he could hurt himself on or fall off of, put in folded up towels and blankets, and lower his UVB and basking lights so that he doesn't have to climb.
See if he will accept some chicken or squash baby food from a dropper - don't force it in his mouth, but drop a little on the tip of his mouth and see if he licks it.
And take him outside in the sunshine. :wink: He will love it.
Good luck, I know it's hard.