Girls, what do you treat with liquid stool in a child? It's been over …

Girls, what do you treat with liquid stool in a child? It's been over a week already. He poops 3-4 times a day, but the smell is terrible throughout the house, straight up, greenish-yellow, watery. We're on a diet, we drink Lactaid. It doesn't help, I thought it was because of the teeth, but it's been going on for quite a while now ?