His birthday/starting school age is not relevant.
Any 'child' at school who turns 18 before taking their A levels can decide to leave without parental consent. So a child who's 18 on 1st Sept can decide not to go back to school, in theory. The compounding issue here is his parents are paying fees (usually these are a term or half a term in advance.)
I find the motive behind your thread @Aspiringhermit hard to grasp.
You say it's a friend but you didn't know about the actual facts about the trust fund (and just happened to find out the truth in the middle of posting this- so you were in conversation with this friend at the same time, last night?)
Then you say you showed the comments to your friend.
Lots of comments are very critical of his parents, so why would you do that?
How do you think your 'friend' feels about having their son and their parenting discussed where anyone in the world can read it?
And what good could you do by writing all of this?
If his parents aren't listening to him, why would they listen to opinions here?
What did you hope to achieve?
If any of this is close to the truth, and he really does want to join the forces, his career teachers at school need to talk to him because if he wants to be commissioned, he'd need A levels and often a degree.