They change things, but he may have had an old child passport which was only valid for 5 years and due to expire? Usually if it's due to expire around the time you're 16, you'd get a ten year one, but you can use a child one until it expires, even if you're over 18 (IME-based on youngest.)
PS Does anyone know (may have missed) what prison will be staging the IS panto? For some reason I want to know this before I can focus on good things, maybe a bit more writing (not in Helen's style and nowhere near as good) getting a new dog (registered with Battersea today) and helping my son get better (not to mention a bit of work). I visited an elderly fraudster in Doncaster some years ago, and met an anti-nuclear friend of a friend outside Lincoln prison in the 80s (he'd cut the fence at Molesworth in an anti-Cruise missile protest) on his release. I would just like to picture where IS will have most of his 30+ years of misery.