Son is a compound sign: first, sign male, then sign baby or infant. To do the male marker sign, use your dominant index finger to touch your forehead, then bring your hand down as you transform it from a pointing hand to an open flat hand.
Transition to the sign for baby by crossing your arms with hands facing up, your dominant arm resting on the non-dominant arm, as if you are cradling an infant.
If your baby is a boy, teach them the son sign when you and/or your spouse is in the room. If your baby's sibling is a girl, sign daughter so that your baby can begin to grasp the difference between them.
Use the son flash card along with the mommy and daddy flash cards. The objective is for your baby to develop self-awareness and understand familial connections. As your child gains further understanding of their surroundings, you can introduce signs for other members of their family and the larger community.