M.B.
Poor little guy. This is a HUGE adjustment for him. The place where he slept for 2 years, which was safe and familiar, is now scary (to him)!
We put our son in a toddler bed at 18 months because he was able to climb out of his crib! We went through an adjustment period and I said the same exact things you are saying now - Did we do this too early, will he ever sleep on his own again! Trust me, this too shall pass.
It took him about 4 nights to really be OK with it. Those 4 nights were terrible, for us the falling asleep part wasn't too bad - it was the staying asleep part that was hard. He would wake up and get out of bed and be standing in the middle of his room in this twilight-like state screaming hysterically.
We tried to make it really fun. Make him feel like a big boy and that it was a huge deal to be sleeping in a big boy bed. For the first 2 nights, I stayed in there with him until he was OK, not until he was sleeping, but until he wouldn't cry when I left. I just couldn't let him be in there screaming his face off all alone - I knew he was just scared! And I went in immediately when he woke up at night because he was literally hysterical. It honestly only took 4 nights for him to get comfortable enough with it to fall asleep and stay asleep w/o any issues.
I should probably mention that we had a fantastic bedtime routine before the transition, so all he had to get used to was the new bed.
Good luck!!!