the phone number associated with an apple id is incorrect

My son is trying to set up a new phone, but he forgot his passord. Unfortunately some time during the last couple years he changed his phone number and did not update his icloud info.

Now when we try and recover his password to his icloud account so he can sign into the new phone it asks for a phone number associated with his account. when we type in the phone number that he is using it says it is not the phone number associated with this account.

thanks,

Chris