Glued box joints would be plenty strong. It doesn't need to support his weight, just the roll-over factor. If it were me I'd make the "L" narrow on the under-mattress side and screw it to a thin piece of plywood (?1/4"?), perhaps 12"-16" wide, to ensure his weight was on the horizontal part before his body pressed outward on the side. Then his own body weight will help hold it in place.
I wouldn't finish the under-bed part, as I'd want it to remain rough to avoid being slippery.
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