I think it is up to the child definetley.
My Dd was in a bed at 12 months, she had been walking since 9 months, she could climb out of her cot and she was ready. She never got up during the night and played etc.

Being that it is a toddler bed they are fairly low to the ground anyway and if you get a safety rail to avoid falls he should be ok but only if you think he is ready.