No matter if it is paid or not, he is helping them.
They offered.
Accept graciously.
Your son, will learn something from it, paid or not.
It is beneficial all around, for everyone. And fair.
He can 'volunteer' for other things. Other things.
This would be his first experience with a 'job.'
That is good.
Do not insist, that he insist, on doing it for free.
Just accept, graciously.
Being gracious, also means, accepting and letting someone offer and decide on payment. That is what they decided and most probably talked about. So it is okay with them. They feel good about it and their decision.
Don't make it a big deal. It may then make it, uncomfortable... for them. And that is not... 'gracious.'
They offered your SON this. They could have hired a stranger or a professional yard man. Which would cost LOTS more. So... they have thought about this... and decided, to offer your SON, this Opportunity.
That is golden.
It will teach your son, about life.
He can also, write them a "thank you" card, for offering him this 'job'... and how he appreciates that.
THAT.. is also being thoughtful and "gracious" and very proper. It will bless them too... the couple that offered him this job.
And, please let your son, decide how to use his earnings.
Teach him about saving... and spending. Of course. But let him decide and learn.
My Mom, has a brother that as a child did handiwork and got paid for it. BUT... his parents took all his earnings. He was not allowed to keep it at all. And he could not, spend it or save it as he wished. He deeply... resented his parents for this. Because... he was not trusted nor allowed to, keep what he earned. And he had worked hard for it.