There are multiple ways to sell a punch.
1. You can punch to the side of his head, opposite the camera angle, without even touching them at all.
2. You can punch them in the head, but right before you hit them stop your fist, resulting in a love-tap.
3. You can punch them with the side of your hand (kind of like a stabbing motion), rather than the knuckles, which doesnt have any bones and therefore hurts less.
4. You can hit them with your forearm, which is very fleshy, and does not hurt.