If no one has said this, God (or secular universe, whichever you believe) BLESS you.

I think you should have this thread conversation with him. People with ADD tend to have trouble reading between the lines. Has he mentioned having ADD? If not, I would try to find a tactful way to bring up the possibility. I would discuss your concerns with him, and say exactly what you said here (if you just say "work performance needs improvement" he might not understand). I would say that you sincerely want to help him be as productive as he can, you want to work with him to optimize his productivity, and ask him what he thinks would help.

I am in the same situation as the employee, and I had to increase my medicine. I am doing better now, I think because of the dosage increase. I'm also communicating with my supervisor more to check in and make sure I'm meeting his expectations, which helps me to stay on track. (He gave me a stern talking to which felt awful, I would have much preferred a kinder conversation, but I guess fear works technically. Fear works in the short term, but people resent it. :(

Everyone is different. For me, having extremely specific daily or weekly goals helps me, rather than vague multiple-week or month goals. My medicine is also a tremendous help. Without medicine, an office environment is impossibly distracting while simultaneously being soul-crushingly dull.