Googled 'degenerative brain disease' but the results were all about the elderly. Googled 'diminished mental capacity'/variations but what I came up with was about how it pertains to law. And I'm not really sure how to go about researching something like this with the specific but broad criteria I have.
Basically, I need an injury or some kind of disease that could cause diminished mental capacity in a previously relatively healthy late teens/early twenties male. It can be caused by any number of things: disease-as previously mentioned, drug OD, car crash, fight, suicide attempt (character has a history of depression), etc. His background is changeable based on what I decide to go with.
I would prefer that he retain relatively functional motor skills, or at least be able to regain them through therapy in the case of an injury. If it's a result of injury I would prefer it not leave him massively crippled, but recovery time being long isn't an issue. In patient/out patient time in a hospital/therapy doesn't matter, actually I'd prefer something that keeps him in it more.
I can do further research on my own, I just really need the basics for the possibilities for this because I have no idea where to start.