Why is it that my husband's breath smells like poop?

Gum infection. It is quite possible that your husband has gingivitis or periodontitis. I would recommend a consultation with a dentist or periodontist. If his gums and supporting structures of the teeth are healthy, one might consider seeing a physician to rule out gastric, adenoid or respiratory issues.
Bacteria. Bacteria, fungus, or other microbes are usually responsible for foul odors. Consider a method that can control or kill the cause and you don't have the effect. One system we found that works is the perio protect method that uses a hydrogen peroxide gel. It has been shown in the jour dent res to kill around 99.98% of the bacteria.
Could be gums. Periodontal disease can cause halitosis. A host of other medical problems can, too. Have his teeth and gums examined by a dentist, and if the problem persists after his dental care is complete, see a physician.