Hi OP
My first thought reading your posts is - if your son is at a prep, surely they are giving you a realistic steer as to which schools he is likely to get into and which would suit him?
Realistically, if the school are telling you he is on a trajectory for 5s and 6 / 7s at GCSE, he will simply not get into the London Day Schools - St Paul's, Latymer Upper, Kings Wimbledon, CLBS, Hampton, Highgate etc.
In these schools, virtually nobody gets a 6 at GCSE. It's like 95% 9-7 every year.
I think your husband may be labouring under a delusion that entry into these schools is on a par with when he was there, 20 or 30 years ago. I think he (and his family) need a reality check.
It makes no difference where his family may or may not have gone to school. There is nothing any prep school can do to 'get him in.' Those days are long gone.
Boys who sit for those type of schools are top of their primary or prep schools across the board. And even then, the majority will be rejected. The competition in London is like nowhere else and its insane.
If you go and look round the schools these days, you will see that the majority are children of European, Indian, Chinese or other non 100% British-born parents - all on trajectories for top grades.
Sorry if this sounds harsh, but I've seen too many dads who assume their kids will go into the same schools they went to. But it's a different world now. It doesn't make one jot of difference as to where he thinks his son should go - it's all about where he will actually get in.
I know this because I have had 2 DC go through 2 of the super-selective day schools. I have another one who, like your DS, was in a trajectory for 5s, but she was / is highly creative. She had to travel quite far out of Central London to find a school she could get into a 11 plus (boarding was not something we would consider). She did end up with average grades. But - she went to art school, then uni and is now having an amazing time on a placement year in Japan. Her course has been just as exciting and celebrated as the other two who ended up at Oxbridge. I still have one to go.
If I were you, I'd have a reality check with your DH and make sure he's actually listening to the advice of the prep. There are schools that will get the best out of children who are more 'average' in terms of academic profile, but still enable them to shine in other areas. Don't let your DH insist on your DS sitting exams for schools he won't get into ( even with intensive tutoring) simply because he himself went there. The prep will likely refuse to write him a reference anyway if they don't support the application. Find a more all-rounder school and let your son flourish in his own way. As long as he's happy, he'll do great. Good luck!