The only rule I've heard about switching to a bed is when they can climb out of it and can get hurt from the fall. IF he is still comfortable in it, why not let him stay in it?
The transition to a bed varies from child to child. Some have no trouble others will see it as unrestricted freedom and can get out of the bed constantly.
Our neighbor had her little boy in a crib until he was 3yo and he's a big boy. She just didn't want to continuely put him back to bed at night.
