Frankly, until we can see TfL documents, no real advice can be given. He cannot appeal any penalty charge notice as the process is at its final stage for debt recovery. The only way to revert the process is to submit an Out-of-Time Statutory Declaration. What would be put on that declaration ?
https://www.gov.uk/government/publications/form-pe3-challenge-an-unpaid-penalty-charge-notice
and also
https://www.gov.uk/government/publications/form-pe2-application-to-file-a-statutory-declaration-out-of-time

Well, it doesn't affect you, it is his problem registering a car at an address that is not his usual address thus not getting mail on time or even at all. However, as you've told us, you are getting a load of hassle from the bailiffs. In the end, the bailiffs will track him down and seize property, usually the car as this is easy to find.

The document you have received is likely to be the Notice to Owner. Please post it up here or if it's something else, post that up. At NtO stage, the discount option has gone, so it's a no-brainer to submit reps again. When they reject, they may re-offer the discount, but are under no legal duty to do so.

So here is the entrance to Horseleydown Lane from the direction you entered: -
https://www.google.com/maps/@51.5027646,-0.0767925,3a,75y,8h,87.78t/data=!3m6!1e1!3m4!1sM5OGwKqyXiSVrppSVCrGzw!2e0!7i16384!8i8192?authuser=0&entry=ttu
Presumably you failed to notice the CPZ sign. The hours on it are the single-yellow line restricted times. However, you've blanked-out the contravention time on the PCN, so we cannot compare it to the CPZ times. So why did you do this ?

I think his post should have been passed onto him.
Only "should" ? Why is there doubt ?

This morning a bailiff turned up at my home asking for my brother, but he hasn't lived here for years.

V5 for his car is registered to my home address.

So was mail from TfL arriving at your address, passed to him ? The only place TfL get the name and address from is by enquiring from DVLA, quoting the car registration number.

That is a vey helpful letter, and advises you to wait for the OfR, so that you can submit a WS to TEC. Submitting the WS is a mere tick-box exercise and gets the process reverted to the original PCN stage at which point you can appeal or pay.

At the end of the period allowed on the CC for payment, the council can register the debt at the Traffic Enforcement Centre (TEC), following which they can issue you with an Order for Recovery. At this stage a Witness Statement can be submitted stating that the PCN was not received. The form to use is the TE9 : -
https://www.gov.uk/government/publications/form-te9-dart-charge-mersey-gateway-clear-air-zones-challenge-a-traffic-enforcement-order
As you didn't receive the original PCN, it would be prudent to now be proactive, and to contact TEC quoting the PCN Number after the period for payment of the CC has expired, to find out if the debt has been registered. You most definitely do not want to miss this stage. You can submit a WS even though you may not have received the OfR. About every 7-10 days is normally sufficient.

When it comes to filling in the form, do note that the "Applicant" is the council, not you. You are the "Respondent".

Finally, is your address on the CC correct in all details and the same as your V5 ? Small errors can sometimes lead to mail going astray.

Well, this does indeed seem to be an error by the council. So what you should do now is submit representations that the offence did not occur, and include a photo of the sign.

I looked on GSV, but it's not up-to-date, latest is 2021 for Rangefield Road. Presumably this is the school the restrictions are for: -
https://www.google.com/maps/@51.4237609,0.0031889,3a,76.1y,67.56h,90.34t/data=!3m6!1e1!3m4!1sYMIWz-_Q0FIMIAnzHFCZbQ!2e0!7i16384!8i8192?authuser=0&entry=ttu