Thank you for your lovely comments! Just to put your minds at rest, Ozzie was handed into the RSPCA because his previous owner became pregnant and couldn't look after him properly. He has never been mistreated or not looked after properly (although his coat is a bit matted because he can't be brushed every day at the shelter, but we are working on that).
He does require coat maintenance, and as he has hip displacia he will require ongoing treatment as he gets older. Perhaps his previous owners, with a new baby on the way, felt they couldn't give him the time and care he will need.
So, no hidden problems, nothing they weren't telling us, and no issues with him at all. But he is a lovely natured fellow with no hang ups at all, and we were very lucky to get him. We consider ourselves very blessed!