This is story related and cannot be missed. You’ll have to fight the powerful enemies near him so make sure to prepare by taking our enemies silently and picking your (probably elevated) position. After you’ve cleared the enemies around him just click on him. You’ll get a cutscene and he’ll be a party member afterward and the achievement will pop. Click the spoiler tag for location.
*** Spoiler - click to reveal ***He’s in the House of Bones (ruined elementary school area). He comes with a Rambino which you may wish to upgrade and equip to an active party member if you need a longer range weapon but want multiple shots.