You do not need to know his regiment to order a regimental/overseas BMD certificate. You just give the details given next to his name - volume, page. year (?) - and apply here:
https://www.gro.gov.uk/gro/content/certificates/most_customers_want_to_know.asp#OverseasIf he died while serving the army would have destroyed his (WO97) record 20 years later. However it is always worth checking FindMyPast as he may have served in the Militia before becoming a Regular, and FindMyPast should have his WO96 papers.
Ken