ROC for k-2 stepson filing separate from mother- supporting evidence?

[kikiyut 1]

good evening

(i searched the forum for this but received 0 hits for "k-2".)

my stepson arrived 4 months after my fiance, he on a k-2 she on a k-1. she and i have been happily married for 2++ years, we filed her I-751 in July, now its time for my stepson's I-751. i am unsure what supporting information we need to include with his filing, as it is separate from his mother's application. for his mother/my wife's i-751, we needed to prove our bona-fide relationship, but for his i assume we only need to show he lives with us and is our dependant, correct?

so far we have : copies of his green card, social security card, selective service registration, school ID, birth certificate and certified translation,  my tax transcripts for last 3 years with him as dependant hilighted, a page of photos of him with us, all three of our health insurance cards (same insurance), a dentist bill (one of many) and a doctor bill, and a bank statement for an account he has jointly with his mother (my wife).

do i need more? do i need to include all the supporting evidence we assembled for his mother? he has his father's last name, i assume if that needed to be addressed they would not have issued him a k-2 visa in the first place, right?

did i miss anything? further thoughts?

thank you