It doesn't necessarily have to be a disability. Because disabilities vary from how serious or innocent they are. Like asthma for example. It can either be almost harmless to someone, or very severe. ADHD, Germaphobe, Arachnophobia, etc. Not actual disabilities, but.... Something. If he's scared of dying, he could be scared of being buried alive? But don't take disability as a offensive word, it's not meant to be. But you can think of somethin' :)