First, whether you have any relationship with him or even ever met him is not relevant to getting his IR2 visa. Don't know where anybody got that idea. The bona fide relationship issue is only related to the spouse, not stepchildren.
Your husband can file now and upgrade his petition when his Naturalization is complete. Sponsorship comes at the NVC stage, so he can sponsor his son himself. That seems the best option to me.
You could petition the son, and your husband would be your "household member" when it comes to sponsoring.
Another option is for you to make a concurrent filing for the son to adjust status NOW, and stay here. Then he can become a citizen once he has a green card and your husband naturalizes. Might be some timing issues there, but citizenship cancels any sponsorship obligations. Of course, he's still your husband's son, so real life responsibilities come into play.