All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad
is His Slave and Messenger.
We ask Allah to guide your son, our children, and the children of the Muslims, and to keep them steadfast upon His religion.
You should begin with supplication, for the supplication of parents is answered. It is reported from Abu Hurayrah
that the Messenger of Allah ﷺ said: “Three supplications are undoubtedly answered: the supplication of a parent for his child, the supplication of the oppressed, and the supplication... ” [Ahmad]
You should also understand that calling to the religion requires patience and wisdom. Allah, the Most High, says: {Invite to the way of your Lord with wisdom and good instruction, and argue with them in a manner that is best.} [Quran 16:125]
Therefore, approach him in a gradual and wise manner, with gentleness and compassion. You have in Luqman an example, when he addressed his son with the language of a sincere and caring father: {And when Luqman said to his son while advising him, ‘O my son, do not associate anything with Allah. Indeed, associating partners with Him is a great form of oppression.’} [Quran 31:13]
You should also try to identify the reasons behind his distancing himself from the religion.
There may be doubts or misconceptions that need clarification. In that case, seek assistance from qualified scholars and callers to Islam, or guide him to reliable and specialized resources.
It may also be that his distance from the religion is due to bad company. In that case, try to distance him from such companions and replace them with righteous companions, so that they may serve as a good example and he may be positively influenced by righteous companionship.
Allah Knows best.