What Causes Strong Smelling Stools?

Greetings, Dr. Rynne, and thanks for taking this question. My three year old Son has horrible smelling feces, and he is also not willing to master the art of Potty Training yet, and often tried to poop in the bathtub when I bathe him daily. Any insights; his feces is always smelling so bad, and I have never had tis problem with my other three children, ever. Thanks for your insights, Dr. Rynne. Sincerely, Ivana, Age 41