He probably would be helped by some
education from a scientific source about thunder, lightning and storms. Obviously, not every cloudy day will produce a thunderstorm, and education is a really helpful tool when dealing with phobias. It is valid to be worried about lightning in some geographic locations -- have you ever watched the college baseball finals in Omaha? They have to clear the field if lightning is in the vicinity, even if it is quite a ways away, for a half hour.