I can't find anything in Stripchat's model rules about no pee. But it does say no poop, vomit, or blood.
Also, do you already have a premade pee video, and you're just thinking of uploading it for sale to your page? Or would this be a custom video you'd have to make for this guy? If he's asking you to make a pee video for him, make sure you charge him a custom video price.