S.O.
One suggestion I got when we were going thru this was to change things just 15 minutes at a time. For the first couple nights put him to bed 15 minutes later than usual. Or at his nap. I did this when transitioning from 2 naps to 1. I did it 3 days at a time. Meaning the first night I moved his morning nap back 15 min, then 3 days later I moved it back another 15 min, then 3 days later I did it again. So eventually he was only taking one nap. You could use this with bedtime or nap time. Then he gets time to adjust a little.