Guinea Pig biting hard?

Hello,

I got two male guinea pigs about a month ago. Both of them come up to me when I walk by the cage and squeak for attention.

My one boy is incredibly sweet, and he licks and lightly nibbles my hand. He doesn't like to be picked up however, and he doesn't really like to be petted much. But overall, he's really gentle.

My other boy, however, is not as sweet. When I put my fingers near him, he bites. HARD. When he bites, he tries to drag my finger back and I have to twist my hand to get him to let go, because he refuses to let go of my finger. He's successfully bitten me three times so far in the last two weeks, two out of those three times it bled. He's tried to bite my dad before aswell. I don't know what suddenly started this. He never did it before, but now any time you put your hand near him, he tries to bite a chunk of your finger off. Is there anything I can do to try and get him to stop biting, or is it just part of his nature? I know it's rare for a guinea pig to bite hard enough to draw blood. I don't even do anything to make him angry. He always starts squeaking for attention, only to bite me. I try hand feeding him treats but he'll only eat a couple and then bite me.

He doesn't chatter his teeth at me and doesn't seem to behave aggressively (to people or to his brother) at all.

My pigs are about 3 or 4 months old.

Any help is appreciated.