I can tell you already know the right thing to do - DON'T hold him back
You're on the right track. If your child missed 4 months of short vowels - then your teacher needs to send him home with that work and you can work with him at home on it. He will pick it up even if you didn't teach it.
It's proven - when kids are worried about where they're living and if they will be eating today they will NOT focus on school... Once they realize they don't have to worry about all of that (which is NOT right when they're placed in a new home but after several months/years of being in a safe home) they'll focus on school and catch up.