The easiest way is to look for any documents relating to their service and tours, have you any of the birth certificates of children that may have been born during their service career? Maybe he was in the army when he married, in which case it might be on his marriage certificate. If he was discharged to pension or discharged to the reserves his army pension book may still survive and that gives details of when and where he esigned up aswell as his physical description and service history and medals awarded.
If he did serve in WW1 then he will have been entitled to a number of medals, these may survive in the family and will give his service number on the rim which helps search for his record at Kew.
Have you a photo of him in uniform? if so the cap badge and other insignia would denote his regiment and rank.
look at
Records are at www.nationalarchives.gov.uk and cover medal rolls and pension records plus there is alot of info on how to search for individuals.
It is also sometimes worth remembering that young men enlisted as opposed to being conscripoted, which started later in the war. Those who enlisted could do so locally, my granbdfather went to Halifax and enlisted with a Yorkshire regiment, he was only 17 too. If you know where the person lived then i would look at the regiments that may have carried out enlistment locally.
It is easier to find someone who was killed or missing in action during the first world war than someone who survived but pension records are probably your best bet. The discharge to reserve can be a good source as the men were only finally discharged to pension as late as 1930 as many of them were called back to duty during the 1928 strikes.