No, they aren't poisonous AFAIK. It will probably come out the other end whole, though. I am assuming the acorn didn't get stuck in his throat and isn't likely to get stuck anywhere in between, though?
My puppy has managed to eat dirt, pebbles, hay, grass, cat poop, acorns, walnuts, hickory nuts, all sorts of leaves and tree bark, deer poop, chicken feathers, chicken poop, layer pellets, birdseed, string, and glittery cat toys--despite being fed the most expensive organic holistic puppy food that can be purchased within ten miles of my house. All came out the other end uneventfully. Well, there was that one time a piece of hay got stuck in his rear and he tried to run away from his own behind, but apart from getting worms after eating the deer poop, he's fine.