The medal ribbon looks like the 1914 Star or the 1914-15 Star.
The 1914 Star was established in April 1917. The 1914-15 Star was established in December 1918. The ribbons are identical but the 1914 Star ribbon had a metal rosette (I don't know if the rosette was introduced after December 1918 to avoid confusion).
William Kennedy would also have been awarded the British War Medal and Victory Medal (which were established in July 1919 and September 1919 respectively). These are not shown in the photograph, which suggests it was taken before he received the medals.
NOTE: it would have taken officials quite some time to compile medal rolls and arrange for medals to be struck, have names engraved, and be posted to recipients!
So, a fairly tight date for the photograph can be given. If the medal is the 1914 Star the photo was taken after Spring 1917 and before William received the BWM and VM (probably before the end of 1920). If the ribbon is the 1914-15 Star the photograph was taken in 1920 plus or minus a year.
Philip