Research supervisor is easily distracted in meetings with me
Most people can concentrate better when they are still fresh, so my suggestion is try to schedule a meeting with him early in the morning, or as early as possible. Like the other answer has suggested, try to keep the meeting as compact as possible. Also, you might also have to try to keep him occupied on you so that he don't have any time to start thinking of something else. This can be done by keeping the discussion on. You have to learn to write while thinking and talking at the same time.