C.
Gate at the door and room pretty empty so as boring and safe as possible...almost like the whole room is one big crib. Can put things like books and such back in over time as the big boy bed becomes "normal" for him.
Mattress out of the crib at least, so going back to the crib is not an option (doesn't look comfy without a mattress).
And of course talking it up with him. :)