The Act identified the following as "reserved" at the start of 1916 (see newspaper snippet dated 22 Nov 1915). These were modified as the war progressed - for example in Feb 1917 the following were ruled out as reserved
"COAL MINING, men who have entered coal mining since 15 Aug 1915" ie. lacking experience.
Your grandfather had the skills to qualify for a reserved occupation, albeit in a wholly different field.
I think electricians in shipyards, directly employed on building ships rather than the yard in general, also enjoyed reserve status.