Just bringing this to the front again. Haven't been able to break down the brick wall on this one. Just posting again if there are any new sources available that I may have overlooked or not be aware of.
The only record I have found since, is his attempt to enlist in the Royal Irish Fusiliers in 1906, when he was clearly underage by a year, give is birth date as 1889 and later was discharged. He used only William Walker on enlistment.
Details of Militia Attestation Papers, 1800-1915:- William Walker, Gender Male, Marital Status Single, Age 17, Birth Year abt 1889, Birth Place Tullylish, Gilford, Down, Ireland, Enlistment Date 5 Nov 1906, Enlistment Place Portadown, Ireland, Regiment 87th Foot (R. Irish Fusiliers), Service Number 4782, Father James Walker, Mother Jane Eliza.