Greetings, and a warm welcome to Rootschat.
The way forward would be:
1) Trace the sons on the 1911 Census and see where they were living, whether or not they were scholars or employed, and what was their father's occupation. (Depending on their social status this may help determine if they may have been officers or other ranks).
2) Check the Absent Voters Register 1918 (available on paysites) to see if they were registered to vote in their local area.
3) Check the Medal Roll Index Cards (available on paysites) for any service medals issued - the cards would give dates, ranks, and (for the Army) regiments or corps
4) If any of them were "other ranks" there is a possibility that their service records still exist (although many were destroyed by fire). If they were officers their appointments and promotions will be found in The Gazette (the official public record searchable free online).
5) If any of them were married during the war their occupations listed on the marriage certificates might show them in the Armed Forces.
Good hunting!
Philip
