How do you stop a friendly socialable dog from being "spooked" by people in headgear.

We've had Scorrie since September of last year and he still doesnt "trust" people who have headgear on .

Tonight the sailing club were down at the pier and a younge lad in a wet suit got the stand off bark but no one else did.
The only diffrence about him is that he had on a cap.

Luckily the lad wasnt frightened and thought our big muppet was funny and he really does look like a muppet.