He needs to be removed from the home.
Not all rescue’s will take him, with bite history but you have to be honest about it and persevere until you do find one.
As others have said, some dogs just can’t cope with children. It’s not about how much you socialise them together, they just can’t. I assume when I see dogs at rescue’s who can’t be rehomed with children, that there is some kind of bite history or at least significant anxiety issues at play that make biting a risk.
I’m really sorry this has happened, it’s so hard. It’s good he showed inhibition when biting and I hope your 9yo is ok.
I had an anxious dog years ago who snapped at my newly mobile baby and then nipped a neighbour the next day. It was a horrible, stressful moment in time. She was a beautiful thing, young too but so terribly anxious, we didn’t even realise until the baby came along that she was. She was rehomed to friends who had space, time, experience with anxious dogs and crucially for her - no children. She’s been with them 7 years now, there has never been another incident, she lives a happy life. Far happier than if she’d stayed with me. It would have been a tragedy to put her to sleep, something my unlucky but lovely neighbour agreed on.