D.W.
I have a different perspective than the other moms. We have a recently turned 2 year old and an almost 4 year old. The younger is a great sleeper, the older is terrible.
we've had lots of conversations with our pediatrician about sleep. It's very common for children, as their growth and development change, to have dramatic changes in their sleep patterns. I wouldn't be surprised if, at 21 months, he's beginning to recognize that he wants to soak in so much more of the world around him and to not want to go to sleep.
That being said, I'd not recommend transitioning him into the toddler bed unless he's trying to climb out of his crib - that could very likely create more issues because of the freedom of being able to get in and out of his own bed.
Good luck.