Yeah...I believe he found you as soon as you got within 5 hexes or so of him.
Maybe if you use the
Spoiler turrets, the Enclave soldiers, or both
, if you can get him to attack THEM, you can sneak up on him and use the items. Not a good idea to do it when he's firing at YOU, though. I always try to maintain a fair distance from him so that the damage he does per attack is as low as 30.