wow! my t3 son has a quarter-size blondish coarse patch on the top of his head. It stands out because his hair is dark brown. It behaves fairly well (but it feels different, almost like grey hairs) and his hair is relatively short so it's not a huge thing.
...Have you tried a gentle color, maybe it'll blend in more?
It's tricky since you've said it's frizzier, a strong dye might dry it out. Have stylists recommended anything for it?
I can tell ds that he's not alone.