If it was his idea to bring you a key, I think he's more mature than you give him credit for.
If the mum being out is a case of his mum just nipping to shops for example, I see nothing wrong with it personally.
We do tend to mollycoddle our children far more than we seemed to be at that age, something of which im guilty of too, but I distinctly remember coming home every day from middle primary school onwards every day to an empty house with my own key as my mum was at work and there simply wasn't any other choice.