When we put our daughter in her toddler bed, she would not stay in it at all at first, and that was ok, really. She would be put down for nap time as usual and I'd leave her in her room knowing full well she was not sleeping in her bed.
The novelty of the new freedom is too much for the kids and they have to get up (wouldn't you! :) She missed a couple nap times due this and when I went in the room the clothes from the dresser were all over the room and the bed was completely unmade. One time, she even took the mattress and angled it off the bed so it was a "slide" (yikes! and she was so proud of herself!).
So I guess I'm saying, don't stress about keeping them in the bed - just resign yourself to an overtired toddler for a couple days - so starting this may not be a time when you want him on his best behavior. He will get over the novelty and go back to his routine. My daughter started sleeping again, but I found her sleeping in all different places, mostly under the bed. I'd just put her in bed while she was sleeping and after awhile she just stayed in bed.
A couple more thoughts though.
~~ Remove anything from the room that can harm or poison him (diaper changing creams etc, breakables, small choking hazards)
~~ Anchor the tall furniture to the wall incase he decides to use it as a ladder and it falls on him
~~ Have you considered how you will keep him in his room at night incase he wakes up before you, so he's not wondering the house unsupervised (we closed the door and used the baby monitor to wake us up when she woke)
I have a son who turned 2 in Oct, and just started trying to fling his leg up (thanks to the tutorial of his big sister), so I've been thinking about this lately too. Good luck!