Originally Posted by AdultKing

He can apply for Right of Abode which will let him live and work in the UK - which is a right you do have if you have British grandparents but is not automatic. But that won't extend to the EU.

Right of Abode is basically what Australian's get in New Zealand, you can travel there, work there, live there but you have no other citizenship rights.

If you are in the Commonwealth, you automatically have Right of Abode and often British Citizenship (depending on when you were born) if a parent was born in the UK and was a UK citizen at the time of your birth.

For more info see:

https://www.gov.uk/right-of-abode