A dislocated should has a 'step' feel to it. If you feel along his shoulder towards the arm is there a little indent downwards?

It's really easy to dislocate young bubs joints. Many many d/ls happen by total accident without you even being aware of it at the time. Because their cartilage/joints is/are still so soft their symptoms are different to an adults.

I'd def get him checked out tomorrow if you're worried.