A Navy seaman from Colfax was sentenced Monday to life in prison for assaulting his 28-day-old son and murdering the infant by sitting on him. Robert J. Howard, 21, currently is posted to the Yokosuka Naval Base in Japan, about 40 miles south of Tokyo, but he likely will serve his sentence at Fort Leavenworth, near Leavenworth, Kan., a Navy spokesman said.