Can I put on Apple pay on iphone of my son(9 years old)?

You must be at least 13 to use Apple Pay but you can use Apple Cash family - To set up Apple Cash Family you must first close any accounts that children already had. Make sure that the balance is zero then go here - Manage your Apple Cash account - Apple Support next, go here - Set up and use Apple Cash Family. (Additional support links View and limit your child or teen's Apple Cash activity, Leave Apple Cash Family, Apple Cash and person to person payments limits)

There is a reason those age limits are in place. Most children that young cannot manage money that way.