Bigglassofwineplease
It's really hard. You can lead a horse to water, and all that.
Can you have a meeting with your son and his head of year? Sometimes a lot comes out in these meetings. And at least you will know exactly what it is he is not doing.
A definite yes to taking away all his devices. Limited use only.
All you can do is keep talking to him. Sometimes it's about really drilling down into what it is that he is finding hard.
That he can't find an effective method to revise, for instance.
Making him understand that it all requires time and application. It's drudge work, but that it pays off.
I also agree that sometimes they coast until their backs are up against the wall. But that's a slippery slope because they get into the habit of coasting. Their knowledge suffers and their work routine doesn't get set in stone.
The problem is, they need their laptops/phone to access things, but then can waste hours online chatting to mates.
Once he realises that actually putting in some work pays dividends, he might get over that hurdle.
Try not to fret. Easy to say, I know. But all you can do is provide as much support as possible. After that it is up to him.
Keep the lines of communication as open as possible. Keep your relationship strong. Try and have a laugh with him on those days when you don't feel like throttling him.
It will be fine One way or another. Whether he does it now, or whether he does it later.
And he may just not be academic. There are plenty of highly successful people who have few qualifications.