Cat with bowel problem

I am writing this for my Mom, she does not compute!
She has an approx. 6-8 mo. old male neutered cat. The last few weeks
he has started screaming when he has a bowel movement, he is not
constipated, occasionally it is a little loose and he gets it all over
the bathroom, he has no other symptoms and plays, acts fine. He has
been to the vet repeatedly and they cannot find a problem and say he
does not scream there. Could anyone give us a clue what is happening,
she is very concerned. Thank you for your help. Stephanie