You could try not calling him anything or addressing him directly at all in public. If you need his attention or wish to show him somthing while you are out shopping, eaither take the objecct off the shelf or rack, approach him and wait for him to turn and then present it to him if possible or go over to him and wait silently for his acknowledgment for you to "speak" with a nod or some other gesture and then tell him you wish to show him whatever it is.
Saying getting your dominants attention without speaking is it's own art.
Saying "Master" with your eyes alone or posture and demeanor may be challenging but isn't impossible.