He's doing this quite a lot at the mo, and the other day he did it in front of the ILs which was fairly embarassing. He gets his willy out and has a fine old time messing about with it; I've just been saying "if you want to do that in your room it's fine but please don't do it in here". But I'm not entirely sure the best way of dealing with it without screwing him up for life, what's the MN verdict?