K.B.
Both my girls started sleeping in a regular twin bed at 18 months old. They were eager to get out of their crib!
I think it's a waste of money to buy a toddler bed because they'll out grow it soon. If you're worried about him falling out - use a railing & lower the bed to the lowest point. My (almost) 3 yr. old has her bed that way. My (almost) 5 yr. old insists on no railing but there have been a couple times she fell out so I just lay some pillows on the floor.
Good Luck!