Hello HuseyinSw
No. The Junior Card requires at least one parent to travel with the child; it cannot be used by another companion.
If there is someone else to travel with your child on a regular basis, you would have to purchase a co-travel card for each person regularly accompanying your child. Thus, you need in addition a co-travel card for your older son / his older brother. It works exactly the same as the Junior Travelcard.
https://www.sbb.ch/en/tickets-offers/travelcards/travelcards-children/children-co-travelcard.html
If it’s just for one travel, you may purchase a Day Pass for Children, given the total travel cost of a half fare ticket exceeds CHF 19. This ticket cab be purchased if your child travels alone or with someone else sponteaneously.
Note, e-tickets purchased for children require an ID card during ticket check. You may purchase it from the machine or counter then your son will not need an ID.
Makes this sense?
Kind regards,
Roland