Lost passport of entry needed for first adult passport

My grandson arrived in the UK when he was 3 and has his UK citizenship registration plus 2 UK child passports. He is applying for his first adult UK passport but they are asking for his original passport of entry from South Africa. His mother threw it away plus her own and her SA ID book and won't help him to get his South African passport. The UK passport office must have seen his original passport of entry plus the Home office must have seen it when he was registered as a UK citizenship. Has anyone else had this problem

Explain that the passport no longer exists.

Note it's not a legal requirement that you must present old foreign passports to be issued a new British passport.
HMPO want to ensure that names across all documents are the same. It is not uncommon that applicants have lost old passports.
As long as British citizenship entitlement is proven there won't be a problem. Documents you are referring to are unlikely to still exist due to Home Office data clear outs - but you still have the registration certificate?

Agree, if your grandson can provide the registration certificate that should be enough. If that is inaccessible or lost, you can obtain a copy here https://www.gov.uk/get-replacement-citi ... ertificate