The son of Boston Police Commissioner Edward F. Davis pleaded guilty to a drunk driving charge Thursday in district court in Plymouth, N.H. Phillip Davis, 22, agreed to a nine-month loss of driving privileges, with the possibility of having six months suspended provided he clears an alcohol screening. He also must pay a fine of $620. In a statement released at the time of the arrest on Feb. 17, New Hampshire state troopers stated that they stopped the vehicle Davis was driving at about 3 a.m. in Plymouth. He was charged with driving while intoxicated, State Police said. Davis has recently been at an in-patient alcohol treatment center in Florida.