I am Dr Mbong. The black stools may be due to GI tack bleeding. However it can be also happen if the patient is taking Iron tablets or some blood tonic or eating a vegetable rich meal.
If it is not due to any of the other alternative causes mentioned above and is a GI tract bleed, then you can make a 'Levy's meal; meal for him.
To make a half a litre preparation of this mixture you need to mix a bottle of anti-acid like peptobismal mixed with 4 ampoules of an anti-gastric secretory drug (Ranitidine
75 mg injectable ampoules) and an ampoule of adrenaline
injectable (hope you are able to find all these ingredients), all diluted in cold
Let him take three tablespoons of this every two hours and you observe what happens.
Try that for a day or two and see if it stops. If it does not, then you will need to see his physician.