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Dammit-
Sorry, missed this at the time. 'Air Mechanic' was one of the Royal Naval Air Service enlisted ranks, so that would be the first place to look; if your great-grandfather was transferred to the RAF he'd also have an RAF service record. The National Archives have some decent online guides to tracing family service members - the RNAS one's here: http://www.nationalarchives.gov.uk/help-with-your-research/research-guides/royal-naval-air-service-ratings/ - which suggests using the search forms here: http://www.nationalarchives.gov.uk/help-with-your-research/research-guides/royal-navy-ratings-service-records-1853-1928/
For any eventual RAF service records, they suggest the best place to look is on Findmypast, which has digitised most of them.
Thrustvector
So AIUI up until the end of the First World War Felixstowe was both an experimental station specialising in flying boat R&D and an operational station that operated seaplanes for anti-submarine duties over the Channel and the North Sea. The experimental station designed a series of Flying Boats that were built under contract by aircraft manufacturers as 'Felixtowe' flying boats. In April 1918, with the formation of the RAF, all Naval air assets were handed over to the new service, and the base became an RAF station. The experimental seaplane work was stopped in 1919, but restarted in 1924 when the RAF's Marine Aircraft Experimental Establishment moved to Felixstowe; they stopped doing their own aircraft designs though. Do you know what rank your great-grandfather was?