OP - you're an engaged parent and care a lot. As does your husband. Your son is very lucky.
Many of the repliers are in the education world, I think, and replying with that hat on.
Tutoring is so expensive and I'm anxious on your behalf that you shouldn't throw money at solutions that will not work unless your son changes his own behaviour.
It's like sport - you will only run a 5k if you can run 30 seconds, then a minute, then 10 minutes without stopping. You can't run a 5k just by paying for a personal trainer and waiting for the results to follow.
Your son is what, 15, 16? It's time for him to start grafting, to make progress towards whatever it is that he wants to do for the longer term.
If he's sporty, could he look at A-level PE? It's quite academic, though - not an 'easy' subject.
There is a lot of work in the sports world.
Otherwise, think of trades or professions or occupations that would suit his strengths, and work systematically in those directions, keeping options open wherever he can, so that he can go 'more academic' or 'more vocational' as and when things change.
Think about what you can do to scaffold your son's learning practice every day. Time on gaming, TV, social media should be rationed - because they're drawing time and attention away from learning and exercise, and perhaps sleep. And they're all spending so much time on them.
Keep checking in with him about how he sees his future. Many young people seem to have a vague sense that they would go into similar jobs as their father or uncle - but doing 'better'. This is especially so since they tend to be better-taught in high school and get better grades than their fathers might have done.
However, it's tougher now for young men to get into good universities compared with their fathers - because it's so much more competitive, and the gender gap in achievement has got so much bigger. And the world of work has changed a lot too.
So, he needs to have a plan, and to be realistic about what it takes to 'do well'.
Whatever his plan is, he needs to be doing a solid couple of hours every evening, at least, on study and homework. He's young and this is the best time to be learning - much easier than learning skills at 30 or 40.
I assume he's in school from 8.30 until about 3. Even allowing for a commute, he has a good 6 hours in the evening to himself.
If he wants a 6, he could certainly catch up if he puts say an hour a day into it - he could use ChatGPT to write a catch-up plan out for him - but he will have to do lots of reading, learning and note-taking.
Actively taking notes, in long-hand, is absolutely crucial.
He may well be very smart, but with slow processing speed. He might be a slow reader, or slow at drafting. If useful to know, screen readers work really well now. But these are sadly reasons why he has to work harder than others - not put in roughly the same amount of time (getting only half as much done) and expect to get the same marks as friends. Even where extra time is made available in exams, or deadline extensions, markers have to mark what is on the page.
Turning up to school every day, even when a bit under the weather, also matters a lot. Every day missed will affect his final grade expectations.
I do believe that training and technique matter a lot, so if he works hard, he will definitely improve. For his GCSEs, this really is the last minute. You may feel that you are doing all that can be expected as a parent - but paying eye-watering amounts for a tutor will just be a sticking plaster if the work hasn't been done.