US Immigration from Belarus

My husband received his permanent residence a few months ago, after a change in status based on our marriage, following a K1 visa.

He had previously been married briefly and had a young child from that marriage. At the time, his ex wife did not want to send the child to the US, and so we did not ask for a K2 or any other immigration benefits for him. We did indicate on my husband's immigration forms that the child existed, of course, and at both the K1 and AOS interviews they asked about him. We just said he was young and for living with his mother in their country, and so we weren't asking for a visa for him at that time.

My husband's ex has now expressed an interest in sending him to live with us permanently. She feels it would be in his best interest, long term, and we are of course happy to have him if she agrees to it.

What would be the best/easiest way to get him a green card or some other legal status in the US? He is still in elementary school. Would we just petition with my husband sponsoring him as a minor child of a green card holder? Or is there something else for the stepchild of a USC?