Hi Ben,
I have a hypothesis which I've not been able to test, because I don't know any nonverbal autistic kids. I know of myself (autistic adult) that I enjoy subject-oriented but usually not person-oriented interaction. So I wonder if it would help if you engaged in a subject or activity that your stepson enjoys. For instance, if he likes to build things with lego blocks, you could build something with legos yourself and see if he gets interested in what you're making. It could be any activity, something he collects, places he likes to check out, a game he plays, a TV show he likes, anything he spends a lot of time paying attention to. Bottom line: your contact with him has to be indirect (running via a subject/activity) rather than direct, which would be uncomfortable for him.
You speak of your stepson as being in a "shell", but from an autistic point of view it's nonautistics who are in a shell or bubble, intimately in touch with each other but not with the real (natural) world, of which human affairs are only a very small part.
Regardless, you seem like a great stepdad, because you care.
Best wishes,
Willem
_________________
There is nothing that is uniquely and invariably human.