Is he actually gnawing, or might he be trying to "suckle?" When I found my first two cats, one of them was VERY young; too young to be separated from mom. He was constantly trying to suckle on my two sons and me. With one son his spot of choice was an earlobe, the other one he liked his bottom lip, and with me it was my eyelid. One morning I even woke up with a little mini-hickey on my eyelid, LOL!
P.S. He did finally stop doing this, thank goodness!