Black stool indigestion

First black stools can (but not always of course) indicate internal bleeding (ulcer, etc.) - so he should be checked thoroughly by a western md to rule out potentially serious conditions. Other medications such as pepto bismol can do it (those that contain bismuth), iron supplements can make the stool black, etc.

So the first step is looking more broadly at the situation rather than the symptoms. Then from a TCM perspective you first (long before you think of what acupuncture points to do) come up with a diagnosis in TCM terms. This is arrived at by using pulse diagnosis, tongue diagnosis, palpation, medical history questions, etc. After you arrive at a diagnosis in TCM terms then you devise a treatment plan. Only by asking lots of relevant questions about history, diet, habits, by using the tools of tcm diagnosis and by experience can you being to answer “why” he has these issues.

The best people to consult would be your acupuncture teachers and work with them as they work through the entire diagnostic process. Acupuncture is not very useful without it being based on a clear and appropriate diagnosis of the underlying issues. Otherwise it becomes a near random exploration of points and that generally won’t yield results.