No real advice but it sounds like how my brother was - he repeated final years of school (in UK) to do A LEVELS again but didn't really do any better - extremely intelligent but not interested - so then he did a foundation year thing (is like a pre uni year for people who didn't get grades for course) did really well and then went on to get a first class BSc and now has very good job. Out of the three of us he has done best for himself but he gave mum and dad heart ache those last few years (he didn't cause trouble etc was just a bit disinterested on everything really). I think also school and doing set subjects didn't interest him but then when did the foundation year in IT he was interested in it so that worked for him.

For some people I think all you can do is support and wait for them to work it out for themselves.