Random question .... Does his passport bio page have to be certified ? It doesn't say that on the uk gov site, but I've seen people say it should be officially notarized ....
The UKVI website used to specify that if the copy was not certified/notarised, it must be accompanied by the UK citizen's original long form birth certificate. However, since they redesigned the website, it no longer says that - and doesn't specify either way anymore.
The suggested list of supporting documents just says 'a copy of', while elsewhere on the website, it says that all documents must be originals (or certified) and that photocopies will not be accepted.
So, my advice has always been to be on the safe side and get the copy certified/notarised. Some people have been okay with just a regular photocopy, but you never know.