I'm pretty sure it isn't a personality disorder. I've dealt with a few people diagnosed with them and they tend to draw people into them and be human wrecking balls. D isn't like that, he just elicits facepalm reactions most of the time.
Honestly, once I started looking around for adult ADHD he spot-on hits just about every single symptom. I'm highly inclined to think that is the case.
There may be some kind of comorbid mood disorder, although it seems like ADHD comes with mood swings on its own, so I'm not sure. I'll have him mention mood disorders to his psychologist when he goes back.
Thanks for the suggestions!