What language does he babble in?

That's a good question! According to Barbara Zurer Pearson, Ph. D., "there is no reflex to say 'gooh'" ("Raising a bilingual child", Living Language, 2008). So, that means that every time Sasha is babbling he is trying to say something or, at least, practicing the sounds he heard. With my linguistic mind in action, I can imagine that, if he is practicing the sounds he heard from us, he should have a nice stew of English and Russian.