R.G.
R., it sounds as though your youngster is already making the transition on his own. Bear in mind, your son doesn't know the difference between his crib and a toddle bed or regular twin-sized bed. The biggest issue little ones have with the transition is losing the bars on the crib, which, when gone, can make them feel less secure in their environmnet. (Oftentimes, the younger you transition to a larger bed the better and the child adapts more readily to the change.) To ease the transition, make sure to use bed rails so your toddler will still feel the sense of containment he had in his crib. Since your baby's crib can be adapted to a toddler bed, you already have the bed rails in place. It should be a fairly easy transition, especially if you let your two year old take part in the conversion of his bed from a baby crib to a toddler bed.
Good luck.