When a baby is exclusively breastfed, his poop is generally very mild smelling and the consistency may be either liquid or cottage cheese-like. Adding in anything else to his diet, such as formula or other supplements, can cause a dramatic change in odor and appearance of poop.
If your baby is getting nothing but breast milk and suddenly develops a lot of mucus or blood in his stool, and the odor is foul, then it is important to have him checked by his doctor. It is possible that he is reacting to a protein in mom's diet — such as cow’s milk — causing his gut to become raw and irritated. It is also possible that he has a bacterial or viral infection that needs further treatment. If he is running a fever or not eating well, then it is important for him to be checked right away. If he is otherwise content, but you're noticing a lot of mucus in his stools, then you may want to keep track of what you're eating, and see if the mucus subsides with a change in your diet. If it does not resolve, or it gets worse, then a trip to your doctor is warranted.