I'm so sorry that happened! When Tucker was young he jumped and landed on his back too. I think it's worse for us than it is them - lol. If he is seemingly OK, then I think you can wait to take him to the vet. If in the morning he's still acting weird, you should probably take him in to get everything checked out. If it doesn't seem to be bothering him, then I wouldn't worry. Sometimes they'll just freak out and yelp like that because they just get scared, even though it makes us feel like we almost killed them!
sending you AND Marley big hugs! Don't feel bad - it happens to most of us at some point!