should I have looked somewhere else for his attestation record?
As he died in service, his attestation document is probably lost. You've looked in WO 12 and it appears that you now have his date and place of enlistment.

The only other place that comes to mind is the collection of Description Books which are mostly in series WO 25. It's usual to find information such as physical description, trade, age and place of birth. This is about as much as you'd find in the actual attestation.

I haven't checked the availability of records for your man's regiment.

Edit: I have now looked at the catalogue and unfortunately it doesn't look very hopeful Sorry.