Girls, son hasn't slept for 3-4 hours past midnight for three days now…

Girls, son hasn't slept for 3-4 hours past midnight for three days now. He doesn't cry, but wants to be held and babbles, reaches out for his hands. He sleeps during the day and evening, but if he can't fall asleep, he gets fussy. We have to put him to sleep and then he won't sleep at night anymore. It wasn't like this before, usually he would go to sleep by 10 pm. What could it be?