I think it is just a normal thing.
Try putting him to sleep a just little earlier (half an hour, 45 mins). He may be over-tired, especially if you have recently switched from 2 naps to 1. I have found that if I put mine to sleep even just a little late for a few nights in a row, it leads to night waking and earlier mornings. A good schedule for this age is bedtime at 6:30 PM, Waking at 7 AM, nap at 11:30 or 12 (at least 1 hour, but 2 is better). If he is whining or throwing temper tantrums, I would bet he isn't getting enough rest. A well rested child is not as emotionally volatile as children that are only sleeping 8 or 9 hours.
They are such little pinballs at this age. He may be working on a new skill--when mine learns anything new (especially big stuff) he loses a little of the other stuff that he already knows for a little while then once the new skill is incorporated suddenly he's back to doing it all. (When he learned to walk, he stopped using his 30-50 words and really freaked me out--he was down to 5-10. After a couple of weeks, he went back to talking.)
He might also be getting some teeth. Tylenol before bed might help. They might be bothering him without really being incredibly painful.
When you go in, don't talk or interact. Don't turn on lights. Comfort him as needed. But keep it as short as possible.
It will pass in a week or so.