I would say 21 monthe is young too warpig, however, what we did, or at least my daughter did, bought one of those little tents that you can get for boys and girls, in our case it looked like a little castle with turrets and a little flag on top, it had a port-cullus door (velcro) we made a game of it, his potty was inside there and it was his special little toilet, he used to go in his "castle" and do the buisness, novelty! but it worked
Good Luck
Bobbi x