If your DB can't get his son to school, he could suggest that they do flexi-schooling for the next year (my guess from what you say is that he is in Y11?). That would involve roughly what you describe: DN works for his exams at home and the school assists (provides exam place, marks coursework, checks on homework).
The downside is that such an arrangement is up to the school. If they say "no", your DB has limited choices - and it would depend on what DN wants to do. Your DB could deregister him from school and home educate until he reaches the end of compulsory school age (last Friday in June after he is 16).
What they do while home educating is then the business of no-one but the man and his son. He could study for exams, but he'd need to find an exam centre as a matter of urgency. Depending on what is stopping the boy from going to school, it might be better to do other things for the next few months and then sign up at a local college for next September and do GCSEs or other qualifications there.
I am posting on behalf of my brother, and have said I will try to provide him with a '10 steps to home education' to help him sort out what he and his son want to do.
In a nutshell, my nephew has ALWAYS had problems with going to school and is now too big for my brother to physically drag there, so if he is too stressed to go, he won't. The school are obviously not very happy about this and still put the responsibility on my brother to get him there and have suggested no other help. In my opinion, the ideal situation would be for my nephew to work from home with the support of school and sit exams etc as planned next summer, but my brother doubts the school will go for any such arrangement.
As far as I am aware, all he has to do is write to the LEA and school and tell them of his intentions, but he would like to know what happens next. There might also be an issue with my ex SIL, who is very manipulative and guilt trips my nephew on all sorts of things. Nephew lives with my brother through his own choice, and obviously in an ideal world he would be able to have councilling to sort out how he feels about his childhood, but he's not in a place to do that right now, so its damage limitation currently. There is obviously a worry that if he home educates, she will make things difficult.
so..... thank you for reading, and if you can point me/him in any helpful directions it would be appreciated.