Can you simply remove the side of the cot and test the theory a bit? Neither of mine went straight into a 'big bed' - both had the side off the cot around 18-20 months. DD1 went into a big bed at 3 (due to DD2 in the works), DD2 is 2 and still in the sideless cot. The only reason I am thinking of moving her is to put them in bunks in the same room.

Might give you some indication as to his inclinations to get out of bed etc, and if it fails, you can simply put the side back on and try again in a few months time.
this is what we have done.
DD went from a sidecar in our room, to a cot with one side off,in the nursery (around her 2nd bday). Next summer she will go in her big girl room with a big girl bed (3.5yo), and DS will go into the nursery(1.5yo)