Thanks for all your suggestions! This afternoon I tried everyone's advice but finally what worked was just rocking him for 5 minutes until he fell asleep, and then rocking for five more minutes to make sure he was good and out, and then transferred him to his crib. He slept for over an hour!
I just don't want to make rocking him a habit.
Poor kid just needs to learn how to sit himself back down after standing and I think we'd be okay.