Men aged 20 to 23 had to register from 21st October 1939, it was a long drawn out process by age group. Forty year olds were only registering in June 1941. So depending when he was killed your man may not yet have registered, or could have registered but was waiting to be called up. They didn’t register and go off into the forces the same day.
National Service (Armed Forces) Act 1939https://btckstorage.blob.core.windows.net/site15176/The%20National%20Service%20Act.pdf