Question Summary:
How do I advise someone to stop committing a sin that I too once committed?

Question Detail:

How do I advice someone to stop doing a sin I used to do myself when I was his age? I’ve repented and stayed away from that sin for a long time but I still feel like a hypocrite for saying anything to him

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Allah Ta’āla says:
كُنْتُمْ خَيْرَ أُمَّةٍ أُخْرِجَتْ لِلنَّاسِ تَأْمُرُوْنَ بِالْمَعْرُوْفِ وَتَنْهَوْنَ عَنِ الْمُنْكَرِ 
“You are the best of nations produced for mankind. You enjoin the right and forbid the evil”
[Surah Al Imran, verse 110]
Your desire to stop others from breaking the command of Allah Ta’āla is a sign of your Īmān.
As members of the ‘Ummah of the beloved Prophet Ṣallallāhu ‘Alayhi Wasallam, it is our duty to enjoin the right and forbid the evil.
Accordingly, it is your duty to advise the individual with wisdom, diplomacy, and love[1], to repent and refrain from committing the sin.
The fact that you have also committed this sin at a point in your life should not bother you or haunt you. You have repented and refrained from that sin; this is a sign that Allah Ta’āla has forgiven you for the sin.
Ḥaḍrat ‘Abdullah ibn Mas’ūd Raḍiyallāhu ‘Anhu narrates that the Prophet Ṣallallāhu ‘Alayhi Wasallam has said:
التَّائِبُ مِنَ الذَّنْبِ كَمَنْ لَا ذَنْبَ لَهُ 
“The one who repents from a sin is like the one who has not committed the sin”[2]
 
In fact, the Fuqahā have mentioned that if one is engaged in a sin and sees someone else committing the same sin, he should still stop the individual from committing the sin. [3]
You should use the fact that you have repented from the sin to your advantage; you have eradicated this sin from your life, thus Allah Ta’āla has rewarded you by allowing you to inspire others to also eradicate this sin from their lives. You can also draw from your experience on what allowed you to repent and refrain from the sin and advise the individual accordingly.
And Allah Ta’āla Knows Best
Mu’ādh Chati
Student Darul Iftaa
Blackburn, England, UK
Checked and Approved by,
Mufti Ebrahim Desai.