If the quest doesn't want to end, talk with Rachel in the healing facility as well.
For completing the quest you will receive some experience. If you have convinced him, go back report to Tina. If the conversation in simple way will fail, you can try again. Pick one of the options (up/down), don't give him the drugs and don't insult him.
You must have high Charisma if you want to convince the boy to get clean. Watch out, as the dialogue is very important and an unwise choice of words might lead to failure. You will find him in the room on the floor, in front of the entrance to the electrical plant.
