I was thinking that this kid's anxiety and need of the familiar sounded so much like Pervasive Developmental Disorder (PDD) - then I checked your sig. It figures!
All I can suggest you do, is try to introduce the toddler bed (or even mattress on the floor) as a second alternative, maybe for play or daytime naps. Give him choice, always - never force the issue. Always make it clear that he can go back to what he feels safe with, you just want him to try the alternative for a minute. Offer a reward for lying on the other bed for a minute. I had to do this with difficult child 1, to even get him to sit on a potty, with clothes on.
I stuck a mini-box of M&Ms to the wall and told him he could have them if he sat on the potty. His clothed rear end barely brushed the plastic and he held his hand out. I let him have them, but I told him that next time he had to sit for a full minute to claim the reward. And so on. It actually took months before he would sit on the potty with bare behind.
Try a similar thing with the bed (maybe with YOUR bed? Lie down on your bed at nap-time and read a book together?) and also offer an inducement for tolerating it, then let him go back to his cot if he insists.
He needs to know he is safe. He needs to know you won't take away the things that make him feel secure. he needs to make the transition, but he needs to know the alternatives and right now he's too terrified to even try.
Yes, it's the Pervasive Developmental Disorder (PDD). That's why you need to introduce to him, the concept of giving something a try, just for a minute. It's like we insist that difficult child 3 try a new food, just a taste. If it's got a texture or flavour we know he won't like we don't insist. Example - tonight easy child 2/difficult child 2 offered difficult child 3 a tangelo. He immediately refused. Then she told him it's like an orange that you can also break into segments like a mandarin. He changed his mind and agreed to have some.
Don't force him, you will make him worse. Just use positive reinforcement (aka bribes, in our house). I bought a bulk box of mini-M&Ms to help me through.
Marg