I'm with Amanda on this one (for the most part). This is more about you than it is your child. It's mommy-guilt. What was working for so long to comfort your child has backfired, and now you are not wanting to deal with the crying and tantrums in order to undo the damage. It sucks, it's hard, but moms are tough and I know you can handle it. After all, you've already started to take some steps.
I agree that you are doing the right thing by transitioning him, but don't be afraid to switch a little faster. The sooner you have an established routine, the sooner things will stop changing and confusing him.
I truly think that, with his age and mindset (stubborn refusal to sleep alone or be comforted by anything but you), he probably ought to sleep in a sturdy crib (mattress all the way down, rails all the way up so he can't climb out) until he's used to sleeping alone. Putting him in the twin sized bed is a novel idea, but is likely to result in you giving in due to his repeated attempts to join you in the big bed. It's easy to give in at 2am after you've gotten up to put him back in bed 30 times. A crib would circumvent that issue altogether.
Stand your ground, give hugs and love, and make him stay in his bed. Sleep in your own bed, not his (another reason a crib might be a good idea, since you can use the excuse that you can't fit in it with him). Offer him a stuffed toy or favorite blanket and put him in.
You might want to start this on a Friday night so as not to inhibit sleep prior to a regular workday. You might even have to put the crib into his room and close the door, since you being visible from the crib might be more of a distraction from learning to self soothe than not. He's going to be ticked. He'll holler and tantrum and make you feel like you are the worst mom in the world...just know that you aren't. If the fits get out of control, go in and pat his back or tell him he's okay, but don't pick him up. He has to learn to self soothe. This is what he needs in order for everyone to be happier in the future, so don't let your guilty conscience get the better of you.
Be consistent and NEVER give in during this transition time. He stays in his bed, and you in yours. You'll find that that consistency will bring change quickly and with less discomfort...but every time you give in, it's two steps backward in your progress. It won't be easy, but remember, this is something you created by allowing it in the first place. It's a hard truth, which means that you have to be the one to take the hard road and straighten things out.
To assist in thinking through some of the guilt which will come of this, a little consideration of future effects: Do you seriously want to deal with an older child of 9 or 10 (or older?!) who refuses to sleep alone, won't go on camping trips with other kids, and is generally a momma's boy because you are too soft-hearted to nip this in the bud now? You are an excellent and loving mom, and taking care of this now will help your child adjust to new situations and being tired far easier than before.
To those moms who are cosleepers and give in to your child using you for comfort instead of teaching them to self-soothe, that's great for you. As a single, working mother, I didn't have time for it, nor did I want the nasty side effects. In my opinion, as the mother of 3 boys, you are creating little self-absorbed tyrants who will be teased and called wussies when they grow older.
Children shouldn't be forced to grow up too fast, no, but keeping them babies only hinders them (and it's selfish, in my opinion). Of course there are exceptions (illness, true fears, owies, praise)...children should be cuddled, loved, snuggled, comforted and so forth, but they should also be taught independence and autonomy, so that when the need arises, they can stand on their own two feet.
We are not raising children to stay children, Mommas, we are raising children to be successful adults. Everything we do as parents should be geared toward that end.
All the best,
C.