Medal Card says Royal Dublin Fusiliers (plus Royal Fusiliers and Labour Corps). He is not listed in CWGC. There are people on the Great War Forum who can help with getting an entry onto CWGC.
I suspect it is a mistake, either by the person who compiled the list for the mason, or the mason himself who have carved RSF a few times.
I cannot find a service record on Ancestry. If he was over 21 you should check the electoral roll. And then you need to check the newspapers.
Ken