my 4 year old boy is scared to poo

Hi, I have a 4 year old boy who is so scared to poo that he poo's only once a week, after he can't hold it in anymore. He runs around the house for days telling me that if he runs, he can keep his poo inside. We went to a pyscologist with him AND to many doctors but nothing helps.He is potty trained from age 2 AND a half already, but when he needs to poo he wants a diaper. The biggest problem is that he needs to poo so i put the diaper on him AND he can hold his poo in for 5-6 days so he spends a lot of time with a diaper on. I don't know what to do anymore AND i am desperate for help!