My understanding is he was in the Durham Light Infantry which did apparently have a Bantam Battalion, but I cannot seem to see which battalion my man was in.
As I'm writing this I had a good further look at all of the pages of his record, and a later page states much clearer his height to be 5'1 1/4" (attached). I think I've found his battalion, the 3rd, which seems to be a reserve battalion.