This is another one of those cases where I started searching for a book, and found where I'd helped someone else out in the process of fleshing out details for the question. I could have sworn someone asked after it on this site, but it looks like it was instead on TVTropes's YKTS (although I can't get it to actually show up on the site anymore, so this is the Google cache).
The book involved a disabled boy in elementary school, with something like muscular dystrophy or cerebral palsy where he has difficulty moving with strength and/or precision. I read it somewhere in the 1990s in Ashland, KY. I don't remember any particular date range, but probably between 1988 and 1994 because that would put me in elementary school and I have this memory of the school library tied to it. The boy had a dragon of ambiguous reality that occasionally seems to help him out, particularly against bullies. There was a scene involving him helping a girl he thought was cute onto a ladder or something where he realized he could see up her skirt, but didn't say anything because she would think he was doing it intentionally. There was a scene involving a hide-out in the woods with the bullies. I want to say there was a scene where he and/or the girl were trapped there by the bullies.
I don't remember how the book ends other than that he was still disabled at the end of the book (no magic cure) and he still wasn't certain if the dragon really existed or if it was just his imagination and coincidence.