Have his parents visit the pediatrician. We had a similar problem with bowel accidents with our son, who had been potty trained for more than a year with no issues. Turned out it was encopresis. It's an issue where the child becomes constipated then holds it in because it's too painful. The obstruction causes leakage, which can appear like a loose stool accident. The child has no control over it and doesn't even realize he/she is having the accident. (Which is why punishing them doesn't work -- been there, done that.) We had to start our son on a stool softener and timed bathroom visits after each meal to get him back on track. It took about a week for everything to clear up as the child has to get everything "flushed out" of his/her system, then get used to the sensation of an appropriate bowel movement again. We also used the reward of a quarter every time he had a successful bathroom visit. We did not scold him for accidents but just focused on positive reinforcement. Apparently, the colon gets stretched out and it takes some time for it to shrink back and for the child to get back to normal. Again, check with the pediatrician as this is a very common occurence in young kids, especially boys. Milk, cheese and bananas can all contribute to the initial constipation problem. Good luck!
This behavior is most likely reflective of strains in his life, and I infer from your comments that he divides his time between two households. Is this correct? If he was truly completely toilet trained and is acting in this regressive fashion, it is indicative of stress. If that is indeed the case, then an evaluation with a pediatric mental health clinician is in order. What do you thnk?