So it turns out that the only possible reason for Sam not being very verbal and not understanding 100% what it means to have an impending sibling is that he is autistic.
How silly of me not to see it. I only spend all of my time with him, whereas the person who told me this has never seen him or in fact heard of him before.
That’ll teach me to ask for recommendations for books/videos/etc. about having a new baby in the family. If he hasn’t absorbed the knowledge by osmosis and magic then he needs a specialist! I will get right on that.
(If anyone has recommendations for preparing an older sibling that don’t involve criticizing me, I am all ears. Or eyes, as the case may be.)