For now I would definitely not try to hand feed him. Often it's not a fear issue, but a confusion issue. They can bite because they think your fingers are a carrot, basically. Hopefully the behavior stops soon. I've not had too many bites through the years, and I usually deserved it by ignoring their cues.
If they are making a rumbly, purring sound they are often telling you they do not like what you're doing. Cavies do not tend to like their bums touched. They are not like cats. You can usually get away with a finger petting the bridge of their nose and head, and usually about halfway down their backs. If you are touching their heads and they are quickly looking up or toward your hand they are telling you no, and trying to push you away.
Good luck.
