There's a lot of messaging that white is the default state in society and biracial kids interpret it as their proximity to whiteness determines their value - and you get what your kid is going through now - a rejection of their heritage/ancestry. Only thing I've seen that works is teaching your kid about their heritage and celebrating it relentlessly - because otherwise it's drowned out by all the other messaging.
Somewhere along the way, your kid has picked up the notion that white is better. Maybe it's all from kids at school - or maybe there are some unexamined ideas/themes being displayed at home through your interactions with your spouse, I can't really say - but I'd venture that's what your child's father's family is probably thinking and maybe it's not the worst idea in the world for them to have another month with him.
How much interaction does he get with folks that aren't white? If he's a quarter of the black population at his school - probably not much, right? Him being a quarter of black kids means there are three other black kids to interact with. Maybe give this book a read?
You might want to post in /r/mixedrace or /r/blackladies for additional feedback.