@goldface
I would start by having a chat now before he starts Year 11. Ask him simply - what does he want to achieve, his ambitions, what does he want from his GCSE year.
( Rather than your ambitions for his grades.)
Another simple question - how does he see himself ? You have described him as a ‘lovely, funny and bright boy’ (which I’m not saying he isn’t btw ) and school reports say he doesn’t put enough work in but how does he feel ?
Let him express that, you may find (as was the case with my Ds1) he didn’t see himself/ability/potential the same as you do.
It’s about finding his motivation and building on that - rather than you wanting him to get motivated IYKWIM ?
My own Ds did not have confidence in his abilities . If he was told he was bright he would deny it as he himself couldn’t see it. It became a stick he would beat himself with.
He didn’t ‘flourish’ or get his self worth in time for his GCSE’s but he has now after his further education and higher.
Once they find their self worth , exam technique, working hard, revision, reward will all follow.
(Acceptance & patience are what I had to give him and faith that it would all work out in the end ) 