he's left-handed
Anyone who's read the Give Yourself Goosebumps books would have suspected this... or at least most of the ones with an "Are you left-handed or right-handed?" choice. :)
I knew about this. When I saw a book about him, I noticed that his watch was on his right hand, as opposed to his left. Therefore, he must have been left-handed. I had a presentation on him, and I am right handed, so during the presentation, I acted left-handed, just like him. My teacher noticed that I wasn't using my right hand like usual, so she asked me why to which, I replied that he was left-handed and I'm just fitting into his role. Extra points!