2 y/o taking forever to fall asleep

So my son just turned 2, 2 days ago. For the last 5 nights, he has taken between 1 and 2 hours to fall asleep at night. Nothing has changed to throw him off so I don't get it. He goes down for a nap at 1 and we do not let him sleep past 4:30. We put him to bed between 7:30 and 8 and he usually falls asleep within 10 minutes. Any ideas as to what might be causing it or what I can do to help. He isn't upset, he just screams and hollers and has a party in his crib.