Here is how you would witness to your confused friends son. You can say, I would tick non binary cos I know In my soul, my innermost being, I am not male or female, Im a child of God and one in Christ. Are you?
The point of Gal 3:28 is not that the male believer is not male or the female believer is not female after he/she is born again.
The point is that whether male or female, there is no preferential treatment between male or female. Both male and female are one in Christ Jesus.
If the point of the verse was that once a person is born again, he/she is no longer male or female, there would be no instruction in later writings concerning what the man is to do or what the woman is to do. But we do see instruction addressed to the male believer and we do see instruction addressed to the female believer in letters written by Paul to the church which were written later than Galatians.
Example: Galatians was written before Ephesians. Ephesians differentiates between the husband (man) and the wife (woman):
Ephesians 5:25 Husbands love your wives even as Christ also loved the church, and gave himself for it;
In the Greek, the word "husbands" is masculine and the word "wives" is feminine.
We are not to change the Word of God to fit some foolishness propounded by those who have been bamboozled into believing the lie of satan and say that Eph 5:25 means that the woman who "identifies" as a "man" (husband) is to love the man who "identifies" as a "woman" (wife).
In Ephesians 5:25 the husband is a man ... not a woman who "identifies" as a man.
In Ephesians 5:25 the wife is a woman ... not a man who "identifies" as a woman.
The world not liking this truth, and choosing to ignore this truth, does not change the truth of God's Word concerning the husband being a man and the wife being a woman.
To change the meaning of God's Word in Gal 3:28 to try to make it fit the parameters of the lie of the adversary results in confusion.