M.M.
I second Mamazita -- twin mattress on the floor is a great first step. Those guardrails (at least the mesh ones that are shaped like an L that slides under the mattress) are a colossal pain when changing sheets, plus I always had to stuff blankets in the cracks and whatnot to keep them snug. If he rolls onto the floor from a twin mattress, no big deal. Starting without guardrails will help him learn to stay in the bed when it is up higher.