Koodos from a fellow Torontonian! I'm also from a post-Sovient country, and I've heard about adopted kids who would shut down / have a meltdown after hearing Russian speech. Unfortunately, in countries like Kazakhstan, there aren't enough funds to take care of the kids properly, due to high corruption rates. As the result, there isn't enough staff and resources to provide children with proper care, which is why babies and toddler raised in orphanages may have some negative memories... I'm guessing it may have been the reason your son used to shut down (but I'm not a specialist, it's just a theory!).
You seems like a nice and kind person. I wish you and your son all the very best!