All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Young people this age need special care and attention by their parents or guardians. So you have to be persistent in advising him. Persistent advice always has a great effect, and its effect could even be immediate. Allaah says (which means): {And enjoin the prayer on your family, and be patient in offering them [i.e. the prayers].}[20:132].
It would be wise to use the method of arousing his interest in performing good deeds and in avoiding evil, and reminding him of texts about the punishment and torture of Allaah. For instance arouse his interest about performing the prayers by explaining to him that Allaah is pleased with those who offer their prayers regularly and constantly, forgives them, and has prepared a great reward for them. Also, you should make him aware that for those who abandon or neglect the prayers, Allaah has prepared a severe punishment and that He is angry with them. If after several attempts he still does not take heed, then you can use a different method, like expressing your dissatisfaction with him and not helping him financially, and ignoring him for a limited period of time, and other means that might be useful to help him. However, you should be careful that you do not push him to befriend bad company. Contrarily, it is an obligation to keep him away from bad company and endeavour to find him a good company, as a person is always affected by his friends and companions. It is confirmed that the Prophet said: "A person is upon the religion of whom he befriends, so you should choose whom you befriend." [Ahmad, At-Tirmithi, Abu Daawood].
Finally, you have to know that the fact that he is not responding to all your attempts of advising him, does not mean that you are sinful like him, or responsible for his sins. Allaah says (which means): {…Your duty is to convey (the Message)…}[42:48]. Allaah also says (which means): {Not upon you, [O Muhammad ], is [responsibility for] their guidance, but Allaah guides whom He wills.}[2:272]. In another verse, Allaah says (which means): {So remind [O Muhammad ]; you are only a reminder. You are not over them a controller.}[88:21-22].
Allaah knows best.