Marmitegirly, my late father in law was also Fleet Air Arm in Ceylon in January 1945. He was ashore with 879 Squadron RNAS and had arrived on the escort carrier HMS Attacker. The East Indies Fleet was being beefed up at the time in readiness for the retaking of Burma and Operation Zipper, the invasion of Malaya planned for August 1945. There were a lot of CVE escort carriers in the East Indies fleet at the time, with their associated RNAS squadrons.
I suspect your Grandfather arrived on one of the escort carriers and then his squadron disembarked ashore on arrival in Ceylon.
The FX & six numbers is his personal number. AM is air mechanic. My father in law had an FX & six digits number, about four thousand earlier than your Grandfather.
Firstly, apply to the relevant MoD department to get his record of service (the link goudie provided should help) . Don't expect too much from this other than his start and finish dates and dates when he was assigned to named units.
Once you have that, do what my wife and I did and make an appointment to spend the day in the archives at the Fleet Air Arm museum. They are tremendous people and will do all they can to help anyone looking to research their relative's FAA service.
Good Luck!