I think his identity has already been verified with Social Security, but if it's not, are there any other documents I can use besides a passport or birth certificate?
If you do not have these identity documents, we can also accept a state issued ID card or an adoption decree.
What if his ID card is in his prior last name? Will it still be accepted?
Yes, we can accept it as long as it's not expired. If we have issues establishing his identity, we will require further information though.