J.C.
I moved both of my kids to a twin bed at 15 months old and never had a sleepless night. You will have to transition him at first since you waited a little longer. I wouldn't make it seem like a big deal. The first day you set it up, take him upstairs and show him the bed, then play in it for awhile. You can read books, color, or just play with toys. Play in it a couple times and it may help him to feel more comfortable when you lay him down that night.