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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
You should continue to offer advice to your son. We ask Allaah to Guide him and Grant you success. You should not despair. Rather, you should continually and excessively advise and admonish him in a kind manner. You should clarify for him the reward of observing the prayer as well as the punishment of abandoning it. Such deeds are good and they are part of giving Da‘wah, which is one of the best deeds and the greatest acts of worship to bring one closer to Allaah. When this is relevant to other people in general, then what about your son, the apple of your eye? Undoubtedly, your task is more asserted and of greater reward. The Prophet, sallallaahu ‘alayhi wa sallam, said: "If Allaah Guides one single man (to Islam) by your efforts, that will be better for you than possessing an abundant number of red camels (a very expensive breed of camels)." [Al-Bukhari and Muslim]
You can use some other means, such as treating him in an unfriendly way, depriving him of your affection, stopping financial support, and so on, if this is beneficial. However, the most important approach is to advise, encourage, intimidate and admonish.Allaah Knows best.