Hi
In WW1 all motorcycle despatch riders were members of Royal Engineers Signal Companies. Whatever their previous experience, they were immediately promoted to Corporal, even on the day that they joined the Army. This was because despatch riders had to be able to speak directly to officers, and the military rules of the day prevented "rankers" (eg, Privates, or Drivers or Sappers).
I'm making a study of the WW1 despatch riders, and if you post your relative's details, I may be able to help you find details of his career.