I have not had this experience before but, maybe you can scream with him a couple of times so that he knows what it sounds like to scream. Try this at home though. Act like it is a game and see what he does. Then when it is your turn again, get a little closer, but not too close and scream in a high pitch, he'll eventually get the picture but this is just a phase he is going through. when the game stops, quiet him a little and maybe even hush him and scream a little softer. Maybe your son will scream softer, if he does while your playing the game, reward him with his favorite juice or cookie. Let me know how this works, it worked for me..Enjoy your day and give your son a big hug and kiss today.
C.