Incontinence is incontinence, and whether bladder or bowel, many of the causes are the same. If the doctors can find nothing wrong with him, then he may simply be going through a growth spurt, and his bowels need to catch up to the rest of his body. Also, consider some of the usual culprits, such as stress, foods, liquids, and in this case, laxatives. As for protection, ask him in a non-threatening way which would be less embarrassing for him... to have an accident while out with his friends, without wearing any protection, or to have it contained within suitable protection. Then, if he chooses the suitable protection, ask him what might be suitable for him. In other words, don't suggest that he wear diapers, but rather lead him there by asking appropriate questions. And, be ready with answers in case he asks you what protection might be appropriate for him. Pullups or briefs, disposable or cloth, plastic pants, etc. may all be possibilities. Consider also Nullo, or other total body deodorizers, to eliminate the odors of incontinence.