It doesn't have to be "either or"; he could get a barky german shepherd AND a good HD shotgun.
I have known a number of tweekers over the years; most of them had firearms or were accustomed to firearms being around them (sometimes with their wigged out friends waving them at imaginary enemies
). If you want to scare them, you have to step outside the norm, even the norm for tweekers. Scary dogs are a natural, wired-in fear, especially for people who spend days or weeks paranoid as heck about "the drug dog" showing up. Many shelters will let you walk a dog around to see how it reacts to you and your kids before you decide. I had this one buddy I had to quit hanging out with when he got too weird, but before that, I noticed the grips on his colt revolver were blotchy/faded in spots; his funky tweeker sweat had messed up the finish severely (I think he slept holding it under his pillow).