How to add a phone number to an existing under 13 family sharing account and apple ID?

My son had an ipod and apple ID that has been under my family sharing account. He now has an iphone and phone number (same apple ID) that I would like to add to his existing apple ID/family sharing account. How do I do this? His account under my family sharing account does not show his phone number, only his icloud email address. I'd like to use the Find My Location feature for his phone but it cannot update if he is not connected to the internet (since his phone number is not attached to his account). Thanks.