Prolonged night wakings

So my son is pretty good about putting himself to sleep at bedtime. We do our routine, put him down awake, and he’s usually asleep within a couple of minutes. But lately he’s been waking up later on...and staying up. Problem is, he wants to be held the whole time. If we put him down, he screams his little head off. When we do give in and hold/rock him, he just stares at us wide awake...for 60-90 minutes. And it’s never at a predictable time! Sometimes (if we’re lucky) it happens at 9 PM. Other times it’s like 3 AM. What is going on?? If he’s able to put himself to sleep at bedtime, why can’t he do it later on? And why is he so awake in the middle of the night all of a sudden?