What you want to do is try teach him that it is not okay to bite. Typically a chinchilla that bites out of aggression will try to get away from you. If your chinchilla comes to you try petting him on the head and away from his mouth and see if he'll tolerate that. When he bites you can tell him "no" push him away, or try to startle him by blowing in his face or tapping his nose. You'll have to do it in a way where he knows it's okay to be around you and okay to be by your hand. He must make the differentiation between getting scolded and not getting scolded. Otherwise he will become hand shy. It might take some experimentation on your part.
I recommend you start just by making sure your hands are well washed and don't smell like food, and put the treat in the flat palm of your hand and see what happens. He might just think your fingers are food. Make sure he is used to you before you start correcting him.
