My 7 year old grandson recently started pooping his pants

My grandson, is 8 and recently started pooping in his pants.  I KNOW THE CAUSE BUT CANT FIGURE OUT THE SOLUTION.  He is afraid the toilet will overflow.  There was an incident about 3 months ago where he was with his mother at a friend's house and after he went poop, she flushed the toilet and the water in the bowl continued to rise and she said the toilet was going to overflow.  It didn't.  We have spent over an hour explaining and showing him how a toilet works, looking into the tank, showing him the water turn-off and having him turn off the water.  We have explained and shown him throughout the house how the plumbing works and told him about plumbers.  We have explained that poop does not cause the toilet to overflow.  When we ask him why he doesn't go in the toilet, he STILL says because the toilet might overflow.  We have tried letting him leave the bathroom and then we flush the toilet and he stills is scared.  HE REALLY IS ANXIOUS about this.  He is not constipated.  We know the cause, as I have said.  What else can we do?

[Kevin Kennedy, Ph.D.]

Rational explanation at this point has gone as far as it can go. From his perspective his fear is not irrational - he has good reason to experience this fear. Be supportive and patient, do not apply any pressure, and establish a reward system for him. The option of not having him flush the toilet for now is a sound plan.