Diarrhea can be serious because they can become seriously dehyrdrated from it. If you absolutely can't get him in until tomorrow, try forcing water into him. You can use either the "Boots tested and not approved" methodd of using a syringe (with the needle off of course) and shoot the water down his throat, or you can, or the "Boots tested and approved" method of adding some tuna water to regular water to get him to drink more fluids.

I sure hope it's nothing serious, but you should definitely get him in ASAP.

Poor Stashy boy, don't worry, your mommy will get you fixed up in no time.