Depending on how bad he is, you could run into real problems getting him to move out prior to closing. I've run into sellers that were relatively motivated to sell, but couldn't get their stuff packed up to move. The volume of stuff is too great to pack in any reasonable period of time. If he has a real disorder, he probably isn't just going to throw everything out. People that get an attachment to things are also likely to overvalue their personal residence. It's their largest possession after all. So, I'm not sure you're more likely to get a good deal from him than anyone else. However, once he has moved on, if it remains full of stuff, then there might be some real possibilities.
Also, if he really has a disorder, he may not have the ability to negotiate with you. You would need to be sure that there isn't someone else that handles his finances. I was involved in a deal where the supposed seller didn't actually have the capacity to sell, because they had been deemed incompetent. That was a mess. Now if I even suspect there is some real impairment, I look into that issue first.
Other than being behind on his HOA, what makes you think he'll be motivated to provide a good deal? Is he to a point where he's in danger of getting foreclosed on by the HOA?