Check the edges of his face and hairline for any blurring or weird shimmering. Does the lighting on his face match the rest of the room? Another red flag is if his voice sounds flat or too clean compared to background noise. Where did your brother get this video, and does he usually mess with editing software?
I get what you're saying about blurring and shimmering... but sometimes that's just from video compression or a bad camera. My brother's videos often look weird because his phone is old. I wouldn't jump to thinking it's edited without more proof.
Yeah, I've seen that happen too with old phones. The compression on older models really messes with the image quality. So it's wise to check other causes before assuming it's edited.