What is the ruling on a young man feigning prayer while in a state of major ritual impurity (janabah) to avoid embarrassing his family, and what is incumbent upon him in this situation?

What you mentioned about the reprehensible acts leads to fornication, and watching pornography, masturbation, and performing prayer without purity are grave reprehensible acts. So, hasten to repent from all of that, and to make up for the prayers you performed without purity.