Probably named after the father of his father. Nothing further known. Most believe that he died young. Definetely not mentioned in the will of his father or his mother.
A record exists showing that a William Hillerd from Little Compton had service in the Sir William Phips' Expedition to Canada in 1690. It shows that under Capt. Samuel Gallup, a body of about 60 men sailed from Plymouth in five Shallops. This battle was won at the expense of many lives. Could one of the lives lost be this William Hillard? If not then, then the participant was his father at the age of 50 years.