the only thig i can suggest is to 'get rid' of the cot if possible, so there is no alternative but the bed. (obviously if you have another baby on the way this won't be possible). or perhaps make the cot unuseable by putting clean clothes or storage items in there.

other then this make it a big deal that he is now a 'big boy' you could even throw a little 'bed party', or some such to make him feel like he is special to be sleeping in a bed.

hope this helps a little and good luck