As I believe you have been informed on other boards (I can recall looking him up myself) the only extant records now are his medal records. I can see the frustration of information available in the past now being lost (his medal card has a 1984 date stamp on it suggesting that someone looked it up then). As you have seen, he is listed as RFA and RHA. Not entirely out of the question for someone to transfer from one to the other.
Frankly, as again someone has already said, the service records from the Great War, if they survived, are on Ancestry and/or FindMyPast, or, as no doubt happens, have got lost somewhere. You'll be aware that the same fate has befallen the records of your Connaught Ranger relative that you posted about recently.
The suggestion was made that his records may be with the post 1921 records still with MOD. I'll check and let you know.
I can't, I lost them in a disastrous computer crash 3 weeks ago and I don't remember now where I got them from!
Suggestion - make a new thread simply asking if someone has the registers of post 1920 service records to please look for his number and/or name. No need to go into the background.
MaxD