I bumped into a friend yesterday, whose eldest DS is in his first year at uni, reading law.
Me: So he got the grades needed, then?
Her: On the button - only what he needed, nothing more...same as always. We're still waiting to see what he's capable of when he tries.
It seems to be a thing with a lot of boys: why break a sweat, when you can do ok without?
DS is in the same mindset. He's really capable, could get 8s and 9s but works at about 75% effort at best and is happy with 7s. He's not in GCSE year, so we're hoping he'll move up a gear when he needs to, but I can't see any way of forcing him to it be doesn't.
That said, the advice about removing distractions, enforcing a homework time, checking planners etc is great. If you know you are doing everything possible to help and support him, the rest is down to him.
You say he is disorganised and hands homework in late. Has he ever been assessed for executive function issues? DD and DH both have executive function issues and both are chaotic, disorganised, poor at planning, can't see deadlines looming etc.
It might be that your son is bright but struggling with organising himself and this might mean he suffers overwhelm, so puts off starting. ( If he's doing homework last thing at night, in a rush, he clearly has some motivation to do it, but perhaps needs a deadline to focus him?)
Just a thought.