I would think it is important to tell the other parents. But even then it is hard to protect your children from coming across material from the unsupervised access of their peers.
You did well to delay smartphone use and to install safety software on other devices in order to supervise his access. Your email highlights how, even when parents take steps to protect their children, they can never fully supervise. As children start puberty they may be very drawn to and excited by watching such material, yet they do not have the maturity to critically evaluate what they are watching. This is, indeed, a tragedy as witnessing such material at young ages can interfere with their normal sex education and development. In surveys many children report coming across this material before their 12th birthday.
He has supervised access to a computer at home where he plays games and watches You Tube and we have parent monitoring software on it. It seems a lot of his schoolmates and friends have phones already and that is where the pressure is coming from. He has been asking us for a smartphone and we have said no, at least not until he has started secondary school next September. Question: My 12-year-old son has always been a well-behaved mannerly boy.