Are the bed rails on the toddler bed detachable? Maybe take them off and see how he does. Most toddlers don't need a bed rail unless there is a risk if they did fall (like off the top of a bunk bed). My DS has slept in a normal bed since he was a baby, never fell off once, ever. Ironically he took a fall off the bed at WDW, because me and my boyfriend were in a full sized bed, and he tried to climb in with us. At home we have a king sized bed, so he'd used to being able to climb in and sleep with us if he wakes up in the middle of the night (which is very rare anymore - he's 5 now).
WDW does have bed rails that can be attached if you really need one though.