It's hard to tell online because you can't hear voice but I sound curious rather than disbelieving with my eyebrows wrinkled trying to think of a way this could have happened. How do you know it's Russian? What were some of the words or phrases? (if you wrote them down). Was it a whole lot, paragraphs or single words? I don't not believe you. What does he say and what's the translation? That would give us context clues to where he might have picked it up. You'd be very surprised how sometimes they can memorize stuff from hearing it once. They're little sponges. I've got a 7 month old who keeps using phrases correctly too often for me to call it a coincidence. She has always had a habit of copying the amount of syllables in a word from 2,3, or 4 syllables. When she chooses not a trained trick. The other day she copied a five syllable beat then quite clearly said "I did it.". (one of her brothers catch phrases he picked up from Dora). I've heard her say "I did it" before. Actually "I deyit" the same way her brother says it plus a couple other phrases in a correct context. But she doesn't talk yet, she just makes baby sounds. Other than the out of nowhere full sentences she uses correctly sometimes. So I have to say she doesn't talk enough to call it talking but too often to call it co-incidence. Luckily she does the same stuff around the hubby so someone else notices.
She memorized stuff. She sings some of the verses of "our song" sometimes, approximating the verse pretty closely. Also she was playing with a toy that said "first there's the egg". She said a while after playing with it "ffff eh eh a". Why am I typing all this here? It has only vaguely to do with the title of the thread and even less to do with what you say your son has done. I ask myself this and answered, maybe to introduce myself and maybe hear more of your story and find out what you're thinking about it happening.
I know that children memorize easily. And my baby copies the tune correctly on the few little bits she's ever sang along to with this as well:

Youth lives by personality, age lives by calculation. -- Aristotle on a calendar