The sign for "son" is a (mutated) compound of "MALE and BABY." It is abbreviated though. The "male" part of the sign just looks like a salute. You abbreviate the BABY portion of the sign (no rocking). It sort of looks like you are saluting from your forehead to the crook of your left elbow. The left arm sometimes just hangs down during the sign SON or at least is more relaxed than in the sign for baby.
