Cassa and others. The idea was that after Brexit a UK citizen could choose to get a UK only passport or get what we have now, which is a UK passport but which in addition allows all the benefits of EU citizenship as well as full UK citizenship.
Going back to a UK only passport is a backward step. A joint UK/EU passport would allow a lot more freedom of travel and the ability to live and work in any EU country without needing any form of authorisation,arrangement, visa,form filling,or negotiation. That is what we have at the moment, but we stand to lose that after Brexit.
Cassa your brother has a UK, EU recognised passport NOT a UK only one. That is the only type of passport it is possible to get at the moment while we are still in the EU.
When we are out of the EU and he needs a new passport he will find that a new UK only passport is valid for a lot less places than the UK / EU recognised one he has now. Of course he will be able to apply for a arrangement /visa of some sort to travel in the EU , but it will be by arrangement or visa NOT BY RIGHT which is my entire point.
There are other advantages to haveing an EU recognised passport. One of them is that if you are in a country somewhere outside the EU and that country does not have a UK embassy but does have an embassy of another EU state. That state is compelled under EU law to offer you the same level of diplomatic protection / representation as it offers to its own citizens. For someone who travels extensively throughout the world that can be a very valuable advantage.