Around these parts, if someone wants my gas and I have a locking cap, they will just move on to the next victim. Even the smallest deterrent will help, so it's not just piece of mind.
We have a neighbor kid like yours too. Kind of the Emo type, with a bit of trouble. I don't let him near my house, his Daddy left him with the grandparents (daddy's a dirtball felon) and the kid has rummaged through neighbor's cars this winter. Sneaky little bugger, I saw his prints in the snow leaving my cars and going under my eaves so as to lose his trail, only then to cross the snow at the back of my property and go straight to his house, I guess he didn't figure I would be up early enough to see the prints in the snow before they got covered.