The United States does not recognize dual citizenship. You received the perspective of the US.
Russia still considers US adoptees as Russian citizens who legally left Russia. They can go back if they wish.
However, unless Russia, like the United States, considers everyone born within their country (even to non-citizens) a citizen, then I am pretty sure they have acknowledged that our son is not a Russian citizens. That is, I think, the very reason they took the step of listing his adoptive parents as his official "birth parents" on our his birth certificate.