In the London Gazette archive is an entry from 1938 -
49th (W. York. R.) A.A. Bn.— Harold Douglas
CONSTANTINE (late Cadet Corpl., Leys Sch.
Contgt., Jun. Div., O.T.C.) to be 2nd Lt.
8th June 1938.
David Stanley Beckett ARMITAGE (late
Cadet Lce.-Corpl., Uppingham Sch. Contgt.,
Jun. Div., O.T.C.) to be 2nd Lt. 8th June
1938.
They were a TA (part-time) unit before WW2 when they would have been embodied (made full-time). If he is your man you should be able to find his promotions from 2nd Lt to Captain in the LG archive.
Ken