Good morning Bella.
You seem to have done a lot of research and probably know that about 60% of WW1 soldiers' records were destroyed in the WW2 blitz.
The details on the medal index card show that he served through the war - not discharged as a result of wounds, or he would have been given a silver wound badge. His date of arrival in France probably
means he was a regular soldier, or a reservist with previous service as a regular, called up on the outbreak. The numbers and letters against the medal entries are references to Medal Rolls, held only at the National Archives at Kew, not online. You will have to visit or get someone to look for you. They may give details of his RFA Brigade and Battery (unit and sub-unit). That could lead to tracing the unit's movements, and possibly to a war diary - again at the National Archives, but some of them are online.
Have you come across this website -
It is very useful for background on WW1 in general, and on tracing individuals.
A driver in the RFA would have been one of those on horseback, riding one of six horses hauling a gun into and out of action, and looking after the horse. This was a less glamorous version of the present-day displays of the King's Troop, Royal Horse Artillery, who appear on the Queen's Birthday Parades and other displays.
Have you looked in FREEBMD for a possible death? That could lead to a date and location.