Honestly, if you have an employee who are under performing for an unknown reason, you need to handle that like you would any other employee. Maybe he already have an ADHD diagnosis that he will disclose to you, or maybe something else is going on. ADHD symptoms exists in a variety of psychiatric and somatic/physical conditions. Just because you are seeing all the signs of ADHD doesn’t mean what that is what’s really going on, and I think that is a very important to remember.

If it does turn out your employer have ADHD you’ve already gotten some good suggestions that you can apply if the times come. But you need to remember that you are his boss not his doctor.