I was going to suggest a rail on the side or a safety gate.
Can he open his door, if not will he sleep with the door shut?

Could you try getting him to sleep on the mattress on the floor so then if he rolls off it's not a big drop and then he can also get use to the bounds of the mattress without worry he's falling out?
We put DD1's mattress on the floor for some time before she went into her bed and she never got out and came to our bed (only now that she can open her door).

GL hun.