Lots of possible reasons why, including that he's a growing boy! The feeding suggestions on the bag are just that -- suggestions. An active teenager may need considerably more, so I'd keep on eye on his weight. And definitely feed at least twice a day, if you aren't already. FWIW, my dog (also a GSD-ish mix) has always eaten considerably more than the bags recommend, and is just now, at three, beginning to seem like she might have finished filling out.
Some of it really is age appropriate -- teenagers can just be jerks. But it's also important to get him on a better track, so here goes with some suggestions.
First, as always, prevent opportunities to rehearse unwanted behaviors. Don't let him have access to edibles (or non-edible edibles), don't let him grab food from your hands, and so on. Control his access until you have taught him to exercise self-control, and be aware that this may be a long process. In other words: crate crate crate crate crate!
Second, help him satisfy his urge to forage. Make him work for his kibble, and enjoy the fact that this is an easy way to provide extra mental stimulation for your dog. Use toys that dispense food (we love our Buster Cube, but there are dozens of options out there), or feed all his meals in Kongs. Or take his food bowl, add just enough water/broth to make things soft, and freeze it. Or throw his food all over the floor. Or feed all his meals by hand, as training treats (more on this in a minute). Be aware when you start out that he probably has very little frustration tolerance right now -- don't make a puzzle too difficult yet, because he'll most likely just give up (one reason people say their dog "just doesn't like Kongs" is that they never taught the dog the value of persistence).
Third, teach him to exercise self-control around food. There are a million ways to help with this, but there is just one that is my favorite place to start: Doggy Zen. All thanks and credit to Leslie McDevitt, whose book Control Unleashed
outlines this much better. It's a great game.
Start with a treat (or piece of kibble). Show it to your dog, and then close your fist around it. Your dog does not get the treat if he paws/slobbers/shoves/barks or otherwise acts pushy toward the treat hand. He does get the treat if he re-directs his attention toward your face, if he adopts a control position (a sit or down), and if he is calm. When you give him the treat, make sure you open your hand and move the treat directly to his mouth -- no opening the hand and then letting him lunge forward and grab it, since that would be counterproductive!
At first, you might need to reward really tiny gestures in the right direction. I think the first time I played with my dog, I rewarded her for one single eye-flick away from the hand and toward my face, even though it was just a microsecond, because that's all she could manage. But you start small and then gradually build, always rewarding average-or-better behaviors (when he regresses, which he will, wait him out). Eventually, you want to be able to pull out a treat (or a sandwich, or a plate of hot dogs for dinner) and have your dog automatically exercise self-control -- sit or lie down, focus his attention on you, and remain calm.
You never say a word, unless you want to mark good behaviors (feel free to say "yes!" when he does something right, or "good boy!" as you hand him the treat). No "off, leave it, no, bad dog, uh uh, sit, down, argh" sorts of things. The idea is that this behavior is cued by the presence of food, not by a specific "command" that you give your dog. It means your dog is going to have to do a lot of thinking, but this is also good. First, because who doesn't want a thoughtful dog, and second, because thinking is hard for teenagers and helps to wear them out
It's a long process, because you aren't teaching him "obedience." You're teaching him manners, and self-control, and frustration tolerance, and all those other really important life skills. The great thing is that those skills are the foundation for all kinds of things in the future -- this particular game becomes a way to teach "leave it," for instance, or a wait-for-release around food. So it's worth doing, even if it takes a while. I hand-fed literally every meal to my dog, in this and other training games, for at least six months after we adopted her...it pays off. And as I said, the bonus with using meals like this, especially if you have a smart and active sort of dog, is that it provides mental stimulation as well as a training foundation. To this day, I feel like handing my dog a bowl full of food is a terrible waste of opportunity
Good luck, and I hope your finger heals soon!