What should I do if I find an unhealthy video in my son's mobile phone recently?

First of all, don't make a fuss, let alone question him. It is easy for children to encounter such problems when they reach puberty. It is not that they are curious about pornographic books, but that they are curious about their physical and physiological changes and can't find a way to understand them. For boys, it's time for fathers to play a role. You can find an opportunity to talk to your child and solve the mystery for him. You can also buy some books on adolescent psychology and physiology and put them on his desk inadvertently. If possible, you can also call your child's school as a parent and suggest them to start some courses about adolescence. Now most middle schools have courses in this field. Don't trust children because of something. Try to get to know him and communicate with him more. I'm sure he will understand your difficulties.