I'm not sure it's advisable to encourage somebody not to do well on their A-levels. It's not like the US where you can get low marks in secondary school and be fine. He won't be able to attend a decent university, ever. Correct me if I'm wrong.
But my advice was to consider eschewing academia entirely. I've worked with too many well-paid, respected developers without degrees to take the value of credentialing very seriously.
It's not a panacea, of course. It takes a lot of sustained work and learning whether you do it in school or not. Some people just find it easier to learn outside of formal schooling.