He may have lied about his birth in order to join up, I cant find a William born about that time in Holbeck. If his DOB is correct he should be on the 1911.
These appear to be the only 2 born in the first quarter of 1911 in the West Riding. If you can find them in the 1911 you may be able to eliminate them.
Roberts William Ecclesall B. 9c 420
Roberts William Huddersfield 9a 269