unexpected side effect of toilet training

An unfortunate and unexpected but, in retrospect, totally predictable, consequence of having started toilet training using a potty seat rather than a little potty: the baby refuses to use the little potty because the big potty is now HIS potty; and, in fact, he bursts into tears whenever Mr. E or I insist on using the toilet, because it is now HIS potty. No matter how much we use the word "OUR".