My mom used to put the twin bed mattress right on the floor so it was easy to adjust to, and she would at first put the crib mattress on top of that so we had all of our familiar bedding. Is he at least in the same room? It might be too many changes at once--maybe put the twin bed in his old room for a while if you switched rooms. My daughter was a great crib sleeper, and wasn't interested in the twin bed--until I let her help pick out new sheets for it. Then it became her favorite place to sleep. It still took her a lot longer to fall asleep than before, but we just made sure anything dangerous was out of reach and waited her out. Eventually sleep wins. ;)
Oh, and small children tend to feel safer in more enclosed spaces, so without the crib rails he may feel too exposed. Child bed "tents", siderails, or even strategically placed pillows or stuffed animals may help create the nest to which he has become accustomed. ;)