I haven't actually visited the memorial at Nebo. I obtained details of it's content some time ago at Gwynedd Archives (can't readily find those notes at the moment)
There is a photo here
http://www.nantlle.com/history-nebo-nasareth-gallery.htmbut unfortunately it's not clear.
Google maps search of Nebo Primary School indicates it's still in use. Street view shows the memorial. I tried to read it but it proved a bit elusive to zero in on.
Do you have family info that identifies him on CWGC etc?
There is no William Roberts to that date but family graves data can often differ.
I'm normally loathe to discuss 'best possibles' but in this case, just in case, the 'best possible' is
http://www.cwgc.org/find-war-dead/casualty/1767773/ROBERTS,%20WILLIAMwhose Soldiers Died Great War entry shows born Carnarvon, enlisted Newport, reside Abertillery, killed in action
25 9 1915.
No papers that I can see to link him to North Wales. It's my experience that Carnarvon in 'where born' can mean the town, outlying villages or even the Shire.