In the above picture, the blue team is attacking. When the ball is played forward by a teammate, no "scoring body part" can be beyond the last line of defense for the opposition (not including keeper).
Being even is onside.
If the attacking team is beyond the last line of defense, and the opposing team plays it back to him, he is onside.
If the goalkeeper is further up the pitch than a defender, he becomes the last line of defense, and he is the line.
Going studs up is an act of aggressive play. Biting people on the pitch is an act of being fuging bat poo crazy. Sorry, biting someone during the course of a game is much more troubling than over aggressive play.
Biting someone is just fuging weird. The way he does it isn't particularly dangerous and it isn't going to leave anyone with an actual injury. It's just really, really fuging weird.
The dude clearly has something very wrong with him. It's an impulse reaction for whatever reason. I'm not going to pretend I even begin to understand adult biting, but I certainly don't think this will be the last time he does it. It's just his reaction to frustration.