Whatever his issue is, it's being helped by either antibiotics and/or steroids. He's walking almost normally and he's not stumbling or falling at all.
I can't tell you how good that feels. I was really getting down at the thought of having to watch him go downhill. I know better than to think that it still might no happen at a later date, but at least for now, he and I both are really happy.
Here's a video that shows him in action -
UPDATE: Teddy doesn't have a tumor or anything so serious. It all seems to be allergy related (something outside). He's now on allergy medication and doing great!