When purchasing tickets, after hitting the Purchase ticket button, you will see the passengers’ name at the top of the list (compare screenshot below: Hans im Glück). There is also a pen icon. Please click on the pen icon to modify the passenger information, including the Travelcards for Switzerland.
Kindly note, tickets purchased via smartphone are personal and non-transferable and are only valid in conjunction with a valid form of ID belonging to the person travelling (passport, identity card, driving licence). You could also order a SwissPass for your son (free of charge), it is also a valid form of ID.
More information on SBB Mobile app and purchasing/using mobile tickets, see: SBB Mobile | SBB help & contact