Hello,
You are correct to be concerned about withholding--it can lead to bladder/kidney infections, constipation, and encopresis (soiling out side of the toilet). Your first stop should be to your pediatrician for an evaluation. The American Academy of Pediatrics has a loose guideline that bowel and bladder control are expected by age 6, so your grandson is significantly delayed in this respect. Elimination and voiding are impacted by both physical and psychological factors. Psychological factors like stress or fear can interfere with normal bowel and bladder functioning.
Your grandson's aggressive behavior around toileting merits the help of a psychologist as well as the pediatrician. You need to implement a positive behavior plan to first reduce and eliminate the aggressive behaviors--him hitting you is absolutely unacceptable and a first priority. A psychologist can help determine why he is resisting so strongly. It may be due to anxiety or perhaps to a sensory integration disorder (some children have extreme reactions to the sensations of eliminating and voiding). The psychologist can create a behavior plan for both home and school to address the issue of teaching him to use the toilet, and can help him learn ways of coping with his discomfort. Prior to your visit, you can get started with the book The Kazdin Method by Dr. Alan Kazdin. This book will help give you an understanding of how to change his behavior in positive ways that really work.
Best Wishes
Rebecca Resnik
Disclaimer: This Medhelp Post is written for informational pursposes only. It is never intended to replace face-to-face psychological and medical care. This post is not intended to create a patient-clinician relationship, nor to give or rule-out a diagnosis.
If I were you, I would seek professional help from a pediatric urologist on this issue. I wouldn't just assume the potting training challenges are related to behavioral issues. Bladder control is a rather complex process and you want to get the right advice early...especially if he's 6 y/o.