How to unlock: After completing the Hole in the Wall quest (M6,3).
Go to the room with the generator on the first floor, near the school. There you will find Tina de Luca, sister of Bobby. She will tell you about his addiction.
You will find him in the room on the floor, in front of the entrance to the electrical plant. Watch out, as the dialogue is very important and an unwise choice of words might lead to failure. You must have high Charisma if you want to convince the boy to get clean. Pick one of the options (up/down), don't give him the drugs and don't insult him. If the conversation in simple way will fail, you can try again.
If you have convinced him, go back report to Tina. For completing the quest you will receive some experience. If the quest doesn't want to end, talk with Rachel in the healing facility as well. If you have very high Charisma, you can convince Tina to move to one of your settlements.
Alternative solution: if you don't have enough Charisma, you can try insulting Bobby. He will attack you. Killing him doesn't cancel the quest and after returning to Tina you will receive experience. None of the inhabitants will have trouble with you for doing that.
Quest rewards: experience, optional: new settler to the village (Tina).