My son started the same problem at the same age. He was still holding it in when he was 16! We spent several years taking him to psychologists, to no avail. I've no idea whether now, at the age of 26, he's any better. It's not the sort of thing I can ask him now! The only thing I can relate it to was the arrival of his baby sister. I think it's quite a common problem, especially with boys. Maybe the medical profession have a better treatment for the problem these days. Hope so. Good luck!