diet

hi

i have just joined the community and i have a 7 yr old asd son who is non verbal he has problems with his bowels and screams in pain i am told that it is overflow and was given movacal which only made his problems worse and this treatment also loosens the elastic of the bowel which isn't ideal. just wondered if anybody had tried diets and if in a similiar situation did  it help or not ? my son has been tested for celiac disease and this came back clear. thanks