If he threw a punch and was not reasonably in a situation where he felt threatened, then yes, he should be charged with assault. However, the person who bit off his finger fails to meet the proportional response requirement in the laws of most states, that is typically you can only respond with deadly force if you are threatened with deadly force, likewise for serious bodily injury. A human bite can cause dangerous infections, it's one of the worst bites you can get. The bite resulted in the severing of an appendage, which exceeds the threat level that the punch of a sixty-five year old average man would justify.