My son also could not remember how to get back onto his back. Even now at 9 1/2 months as he's started to crawl, he cannot, for the life of him figure out how to flop over. He will.
But he used to wake up on his belly and seem to say, "How did I get here?" I stopped worrying about him when I found him sleeping on his stomach. He's not going to smother himself any more at this age. For a while, I wedge some rolled up receiving blankets way down by his hips. The foam wedges sort of scared me. He sort of just stopped waking up crying to find himself on his stomach and adjusted to sleeping through the night on his stomach or side. Like I said, I worried at first, but he just sort of grew out of the panic and eased in to comforting.