J.W.
It could be that he is cold. The weather just started to change, and my son did the same thing. I made sure that I went in and covered him before I went to bed, and he started sleeping again. Did his diaper leak? We had to switch to night-time diapers when our son was 12 months, because he was waking up early. I agree with letting him try to put himself back to sleep if he wakes up early. As long as he can't get out of his crib, I would just leave him. Good luck!