After my son turned 3, we also put a big boy bed in the room along with his crib. He thought the bed was cool (we also got cars sheets and quilt to entice him) but had no interest in actually sleeping in it as long as his familiar, cozy crib was an option. So we removed the crib. No more choices for a 3-year-old. After 2-3 rough nights, all was well. And by rough nights, I mean that we had to snuggle with him until he fell asleep, and if he woke up in the middle of the night, he came and got in bed with us.
One caveat - I read an article by Dr. Rosemond that said there is no hurry to get them out of cribs, and I agreed with it. He suggested that 3-3 1/2 was a good age. That's why we waited until Will was 3. I would say let him stay in the crib until he's at least 3...especially if he's sleeping well in it. Our son is such a good sleeper, and we didn't want to spoil that. Not just 'cause it's nice for us but because sleep is SO important to their health and development!