My Husband Has Had Light Brown Grainy Loose Bowel Movements

My husband has had light brown grainy loose bowel movements for a few months. He had a clear colonoscopy 3 years ago. Should he have it repeated? Our MD says more fiber, which he is eating. Sometimes the movement is mucosy. He is a Truck Driver and is afraid to eat while on the job. No place to stop for bathroom breaks. Please help. Thank you.