Steve Biddulph is one of those pop-psychologists who managed to find a hook that people liked and has based his whole career on it. There possibly is something to his theory but given that you can't find anything else to agree with it then maybe there isn't. Regardless, my mum gave me one of his books and the back flap says that between him and his wife they have multiple qualifications in science, psychology, social work and nursing (without being specific about what science and who has which degrees). The book I have doesn't talk specifically about boys so there's nothing in there about the testosterone theory.