C.W.
You just do it- I would recommend skipping the toddler bed and just going straight to a twin as it gives them more room to sprawl when they sleep. My kids didn't like their toddler beds - when we got them twin beds they were in Heaven.
For encouragement you can tell him now that he is getting to be a big boy it is time for a big boys bed - new comforter can make it fun or a new stuffed animal to sleep with too- I would let him pick them out though..
Hope that helps!