He would have had to be in on the third draft Registration. Prior to that he would have had to be twenty one years of age unless he lied about his age.
The third registration took place on 12 Sept 1918. You had to be born between 11 Sept 1872 and 12 Sept 1900. Prior to the third registration and the supplement to the 2nd registration. You had to be 21 yrs old. He would probably be registered after 12 Sept 1918.