It's up to you really. A child can stay in his crib up to age 4 so long as he fits in it and is sleeping well in it.
When you do decide to make the switch, you can either take the side off his crib and use it like a toddler bed or just put him in a twin bed with a bed rail.
We travel a lot so when our twins were that age, we took the sides off the cribs (because they no longer fit in their pack n plays and needed to learn to sleep on the floor/air matress while away). I would've put them in beds with rails but DH was too nervous. They each fell out of bed once or twice and that was it. We got twin beds in Sept. and they've been doing great.
Either way, the trick is to get him to stay in bed so just keep putting him back or gate his room off if you can. Another option would be just to put his matress on the floor for now. Good luck!
at 1:41 PM on Oct. 21, 2009