Depends what he wants to do if he "needs them".
But I think it's silly to say why does he "need" high grades. He might not "need" them but he will have better options if he gets them. Why does anyone need them? If we all race to the bottom then what good does that do anyone?
He may have ambitions now that don't need them, but later regret it. Far harder to go back. Ever heard someone saying "I regret working for my A in GCSE maths?" No, me neither.
Ever heard someone saying "I need to work and get my GCSE maths to do something I now really want to do, wish I hadn't messed around at school"? Yes, on here and in RL many times.
I remember a thread on another forum about learning a musical instrument and whether parents should encourage practice. It was full of people saying that they'd drifted along and stopped and wished they hadn't. The majority wished that their parents had nagged or insisted they practiced.
None of these people "needed" to play a musical instrument, but in a thread of into the hundreds of replies there were perhaps one or two saying that they hadn't appreciated their parents pushing them to practice.
OP, I agree. In my dc's case it isn't overnight that's any issue. It's the "rush through my homework to get back to my game" or "do my homework with minimal effort because I'm mostly concentrating on talking on Discord."
One of the issues is that such a large proportion of homework is on or needs the computer. It means they've got the temptation right in front of them. It's "just a quick game, only take 20 minutes" and three hours later, they realise they haven't gone back to the homework and it's bedtime. Done the equivalent myself on MN. 
I've given dd a week to show me that she can work in her bedroom without me nagging. So far this evening I have a strong suspicion that she hasn't even done one, she was warned at 3:30, it's now over 2 hours later.
By next week I suspect she will be working where I can see her, as otherwise she never does anything she doesn't have to, and what she does do is with the minimum of effort.