My three year old son has been potty trained for more than 6 months but he won't defecate on the toilet. He wears underwear only day and night and will tell me as soon as he 'does it', but no matter how long he sits on the seat immediately after he won't complete his task! I've taken away every one of his beloved animals, given him showers afterwards( that he hates), but after the second incident it becomes no big deal to him. Any suggestions??