I'm having similar issues with my DS, he's in the middle of his mocks now so we don't have his results but he got 1/2's in the summer.
I just don't know how to motivate him, he does bare minimum at school, can be distributive, not take things seriously. He's started saying he struggles to concentrate and with ADHD in the family it does make me wonder but not able to do much about this in time now for any adjustments. School seem equally as frustrated with behaviour but also that he could do so much better. He's still predicted 4's in most subjects, and a 6 in maths.
Things I've tried without success, he has an English tutor which he started working with in sept, so I'm hoping there will be some improvement there, and that maybe some of the stuff will carry into other subjects as he struggles with getting his ideas down and making it clear, although the subject knowledge of often there.
Brought revision guides, printed past papers, badgered to get him to just do a few pages a day. Offered to support him, do it with him, created a time table. Talked at length about how harder it will be if he doesn't get the 4's he needs. Offered incentives if he passes, I know he's frustrated with me. Even getting him out of bed and to school on time is a challenge.
He's applied for engineering at college and needs a 4 in maths and English for the level 3. Thankfully he's done engineering gcse and construction for which they are using course work so we know if is on track to pass. He seems keen on the course so I'm hoping if he gets on he will switch on once it's something he enjoys.
I'm similar in that all my friends have high achieving kids and my family the same and I'm trying not to focus on how disappointed everyone will be as I know this is my issue, but it's so frustrating that he is capable of so much more.