Sanne, like the others I can really sense your frustration for him, and your love for him. Being in another country I can't offer any practical advice about working within the possible systems available to you... But I have a child who absolutely only engages, learns and functions well when challenged enough (which is often VERY hard to keep up with). And yet lacks the EF to cope with the "boring" challenge of presentation and polishing (ie preparing for a music exam by polishing work which is at the correct level for her to get excellent grades on the exam but below her current interest level for learning music on that instrument). We never know when she will get As and when she will fail. She refuses utterly to use any sort of diary or personal organization system to support her absent EF, and if we push a diary or documentation system she fails more through pure obstreperous behavior. Trying to explain to her that she has to prove she's capable of more for school to understand what she needs is completely pointless... I would say that 8-10 were the worst years. She's 3 years into steadily improving and some of that seems like it must be neurodevelopmental as she goes through puberty. Some of the improvement has definitely been finding the thing that allows continuous challenge and the benefits of that spilling into other areas. I hope you find something soon which helps unlock the stalemate situation you are in.

Asking him what would help, and what he thinks is going on, and really listening (however long that takes) and investigating online college courses sound like excellent advice.