he loves school and that's why he struggles to think beyond it
^ this is what you need to overcome. And one of the reasons IMHO schools should not have VI forms, but that's personal opinion and not much help.
If he does really want to stay there then looking at other options (FE colleges / other VI forms) just with an aim to walk in and say, "At XYZ I can do this combination" and see if they can change things.
He also might be impressed by the facilities available in an FE college.
He does not need A Levels to go to uni unless he is doing something like medicine, or he wants to go to Oxbridge, BTECs are a better prep for doing something like Outdoor/adventure education.
But where does he want to be in 5 years time? If it is sitting in an office then this is not his best option, if it is running an outdoor centre for school trips it will be.
I suggest he looks at possible jobs and works back from there.
If he decides to do BTEC at his VI form with an A Level alongside then it will look better on his CV,