Preventing Guinea pig from nipping?

It's good that you've reached an understanding about when he wants to go back. Don't forget he is young, and he will learn by trying things out with his mouth. Don't put pepper on your hands, it might get in his eyes and hurt him, and he will lose trust. He should grow out of it, but what you could do is purse your lips slightly and blow on him for about half a second. That's what piggies do to each other. It won't hurt him, and it will give him the idea that you want him to stop. Don't blow in his face, just enough that he can feel it, and it will draw his attention.