He has finally transitioned to his toddler bed and I have since taken the crib down completely

I had my kids in twin bed with a bed rail. Or if you're able, have one of the long bed sides against a wall so you only have one bed rail. You could always put pillows around the bed on the floor to add extra cushion if he rolls off.

Smells...
Mainly your's.
Put your pillow case under his sheets so he can smell you.
'Monster Spray'...
Make some 'Monster Spray' so he can use it under his bed and in the closet, and wherever else he thinks monsters hide. It'll build his confidence as well.