G.T.
Interesting. He's 6 years old so it seems you should be able to explain to him how irrational he is being. I don't know your son or his maturity/intellegence level. If it were my 6 yr old I would get graphic on what holding your poop in does to your body, explaining that it is poison waste and thats why it has to come out. I'd show him that the toilet is flushing fine. And I'd make him sit on it for at least 15 minutes and let him know how important it is to go otherwise he'll have to have an enema. But, that's just my sons, they could handle that. If yours is more timid you might need a different approach. I'd load him up with so much fiber and fluid he wont have a choice but to go. The longer he holds it in the harder and drier it will get and will probably hurt when he goes :(