So you have the Half Fare Card then? That was not clear from your original post, where you only mentioned buying Saver Day Passes.
If you added your son to the purchase when buying the Half Fare Card you should have received the Family Card, and in that case you do not need to get a pass (or ticket) for your son. If you forgot to add him, you will still need to get the Family Card. Do that at the station as soon as you are in Switzerland.