Two step verification with lost phone

Hi! My son lost his phone in the lake and I spoke to apple support and the insurance company that handles apple care theft and loss. I have to fill out a claim in order to get him a new phone. He no longer has another device to send a two step verification code to so it won’t let me sign in to fill the form out. How can I get in for him to get this taken care of. He’s only a kid and his Apple ID should be tied to our family plan as well. Should I sign into mine and use the serial number and info they said I needed? Where do we go from here?