This is a crazy age,they got through regressions around 15/16 mths then again closer to 18 mths. Teething, sa and a need to have their A times increased usually causes the problem, which fast becomes and UT/OT loop
Are you doing anything to help with the teething pain?
What did the last to days look like EAS wise, also if he had any NW and how long he took to settle at BT.
Is he an independent sleeper ATM for all sleeps or just naps?