I'm not sure...looks really similar to some stuff I've seen like a purple kale or a broccoli rabe or once there was a purple type of mesculin, but I've never seen green one side and purple on the other side. Curious to see if anyone else knows.
If so, it is toxic, and I'd be curious if it was in the seed mix or got blown/carried into the area. Its good to identify what comes up from a seed mix!
If it is night shade, its toxic. It will eventually have clusters of tiny white flowers that turn into small round green balls. Then the balls turn purple. Each ball has dozens of seeds in it.
If he just ate a little I wouldn't worry. Keep him hydrated, and if you want, you could feed something that might move things along quicker like cactus pads or pumpkin.