Amethyst is right, that it doesn't seem complicated exactly. There are no points in her explanation that seem unclear, or even really gut reaction that says "no way that's impossible." He just thinks about it, and... it's there. It's information. It's an explanation.

The only improbable thing is that all this time passed with no one knowing, but he already knew his mom was a liar.

"She... what?" he barely manages, looking at her with obvious dread. "How... how did you... you didn't know, did you? How did you find out?"

He knows that she just said that she never told anybody, but there is the initial kneejerk fear that all the gems knew this whole time and just never told him for whatever reason. He feels lightheaded, and he sways a little on his feet.