Hi Wendy
I have to agree that the chances of him serving in WW1 is pretty slim, and as already stated he would have needed to serve abroad in order to receive medals.
The seven-digit service numbers were only introduced in the 1920s, which with the use of the term Royal before Tank Corps as well, suggests he served after WW1. If so, his records are still with the MOD and you will have to apply to them.
Ancestry have supposed to have published online all surviving service records for men with surnames beginning “H”, for men who served in WW1 and left before the end of 1920, so unless he is already on that site, applying to the MOD appears your only option. Ancestry appears to be "down" at present, so I cannot check it.
Ken