S.G.
Toddler beds are lower to the ground if they fall off, but they do grow out of them. A twin bed is better because there are all those cute fun sheets to choose from, and if you have an IKEA store anywhere near you they have some pretty neat twin beds. Let him help pick out his bed, give him a couple choices of sheets and he'll feel like a big boy!
I like the other lady's suggestion to put one side against the wall and if you feel like you need a rail on the other side get one. Reward him for staying in his big boy bed.