He may interrupt you constantly. Stop him gently, he is not being rude.
He may ‘blurt’ comments without thinking it through. He’s just saying what everybody else is thinking. Give him a nudge.
He will do several things at once, and not finish any of them. The upside is that he will be a lateral thinker and smarter than most. He will not be dull.
When he is ‘in the zone’ nothing else exists, including you. Let him be.
His conversation will leap all over the place, follow if you can, it will make sense in the end.
Almost nothing will be linear – never expect to go from A to B, unless you are prepared to go via M and F and Q. You’ll still get there. Probably.
He may be very anxious, and sometimes depressed, being ADD is not a walk in the park.
If he loves you, he really will love you!
For the rest, he’s just a bloke like any other.