Sorry I didn't answer sooner, I just noticed this post.
Well, the doctors are very wrong about saying he would have gotten it as a child. "MYTH: Unless you have been diagnosed with ADD/ADHD as a child, you can’t have it as an adult.
FACT: Many adults struggle all their lives with unrecognized ADD/ADHD impairments. They haven’t received help because they assumed that their chronic difficulties, like depression or anxiety, were caused by other impairments that did not respond to usual treatment."
From - http://helpguide.org/articles/add-adhd/adult-adhd-attention-deficit-disorder.htm
This link is worth reading.
You also might want to read this link on 8 Reasons Adult ADD Is So Often Misdiagnosed - http://www.additudemag.com/slideshow/139/slide-1.html?utm_source=eletter&utm_medium=email&utm_campaign=December
You also might want to read or check the reviews on this book, "Is It You, Me or Adult ADHD"? http://www.amazon.com/Stopping-Coaster-Someone-Attention-Disorder/dp/0981548709
It is hard to tell if he has ADHD from your description. The lack of compression is troubling. It is very possible to have both dementia and ADHD. Has he ever shown signs of depression or frustration before this problem with compression?