This is no crackpot. He's a mathematician who has his PhD in the right area.
I am not qualified (in the sense that I have
My guess as an outsider... it will probably turn out to have a few holes in it. But if he's claiming it, he's probably really got something; if it's not there yet, it will probably turn out to be an important step, and if the holes are small, they'll be plugged soon enough. The fact that its a short paper increases the chance that Li hasn't missed something.
h/t Isabel Lugo