As far as I know, and this isn't concrete mind you, you have to "teach" your pawn how to use Debilitating/Blast Arrows. They learn from watching the Arisen fight and try their best to mimic their fighting style, so if you abuse the Special Arrows, your pawn (and any pawns hired) would naturally begin using them as well. However, they are very selective in who "deserves" the Special Arrows.They won't use them on any random enemy like a Goblin or Saurian, but against the likes of Ogres, Garms, Gorecyclopes, Eliminators and such, they will equip them immediately and start going to town. This is all from my experiences with my pawn as well as others. Again, nothing is concrete.