Dog with Bloody Slimey Diahrrea?

My mom's dog has been getting diahrrea and it's slimey like phlegm and sometimes bloody. I've been telling her to take him to the vet because I'm sick of cleaning it... and he walks around while he poops so it goes all over the place and it smells DISGUSTING. What could this be caused by? It's not his food or anything cuz he's always eaten the same kind. If it helps, he's a 7 year old German Shorthaired Pointer.