Totally agree with the advice above...very normal, not a sign of aggression at all, and just be consistent about handling it and he'll grow out of it before you know it. In fact I would probably be more concerned if a 12-week-old puppy never
tried to mouth you or play inappropriately with you! He's still learning his most basic manners, after all.
Another question for you guys. I feed him 3 cups if nutri-source a day and he still is looking for food. Should I switch food brands or is it normal for him to seem hungry after eating?
I wouldn't go by his attitude but by his weight. Many dogs always act like they're starving. Is your puppy a healthy weight and does he have a good energy level? If so, don't increase his food. You don't want an overweight puppy, as extra weight puts increased stress on developing joints and may contribute to joint and bone problems later in life. Of course, you don't want a skinny pup who isn't getting enough nutrients either, so it's a bit of a balancing act.
What does he do to make you think he's still hungry? How often do you feed him? If he really is abnormally fixated on food, it might help to break his feedings up into more, smaller meals so he has less time to feel hungry between meals. It probably wouldn't hurt to ask your vet about it too, if you're in doubt.