Hi!
We had to move Emily (my oldest) out of her bed at 22 months to make way for little Haley. We moved at the same time, so we talked up her new room and her new "big girl bed" all at once. She was pretty good about it, but the first few weeks there was a lot of up and down. We used our gate strategically..at first we placed it at the end of the hallway, but then moved it closer and closer to her door. (We have one of those gates that expands up to five feet and it portable.)
eventually we placed it in her door frame and she figured out that she was supposed to stay put, but it took a good 3 weeks to get her to that realiziation.
Honestly, Haley is 18 months old now and we are contemplating trying the toddler bed with her now. i think it all boils down to how ready your son is and if you think he will understand the concept. Good Luck!