Not HG....
The uniform is as suggested SD.
The Field Service Cap has been in use by the Army since the late 19th century, and when worn in conjunction with SD would date it from 1937-1941/2. Many of the units sent to France with the BEF in 1940 wore SD, until they were issued Battle Dress, some even returned on the ships in SD! I have even seen examples of SD dated to the 1960s!!

There is the hint of colour on the peak of the FSC, but the "V Shape" is not a defining feature, I'm afraid.
As for Regt...Ummmm No badge really to be seen...
Have you a date of birth/death fro him? If so why not ask for his records from the MOD?