The question is whether he is already a US citizen or not, which is a matter of law. If he is a US citizen, then he can get a US passport; if he has insufficient evidence of citizenship, he can try to get more evidence and try again, no matter what age he is. If he is not a US citizen, then he cannot get a US passport no matter how many documents he gives.
INA 320 says that a permanent resident under 18 living in the US in the legal and physical custody of a US citizen parent automatically becomes a US citizen. We need to determine whether those conditions were ever met. He was a permanent resident under 18 living in the US. He had one US citizen parent, you (his mother is not a US citizen). So it depends on whether he was in your legal and physical custody between when he entered as a permanent resident and when he turned 18. From the letter, it sounds like you did not have legal custody. Even if the mother consented to the child living with you, that does not give you legal custody. Joint custody also works for the purposes of this law, but it sounds like you did not even have joint custody.
So he is not currently a US citizen, and would have to apply for naturalization on his own after he is a permanent resident for 5 years.