Yes, "might have been poured concrete" does mean that he was immobile like a poured-concrete cast. 

The sentence could use a comma:


  After he hit it, he might ...


Also the part “been poured concrete” seems odd. I was half expecting it to end “... he might have been poured out and shaken up for all he could remember.”  In other words, coming after been, I tended to parse poured as a verb at first; its use as an adjective threw me briefly. 

But I can't think how one would improve that except by being more verbose, as in:


  ... have been made of poured concrete ...


That's two missing words, rather than one, and it seems to weaken the directness of the author's style. 

Any omitted word in this sentence?
I'm learning the English language, and while reading Merriam-Webster I often see common words with additional "obsolete" and "archaic" descriptions added to their definitions. 

When should I use them, should I use them at all, and what's the difference between these descriptions? Also, should I spend time to remember these archaic and obsolete meanings? 
