How long as he be experiencing this constipation and dark color?

In normal situations, the color is determined by diet. If his stool is BLACK, then there is cause for concern, because it would mean that there is blood in his stool. I do not think this is the case here.

To alleviate constipation, have him eat less constipating foods such as bananas, cheese, processed flour and foods, etc. Put him on a diet that is higher in fiber with juices, especially prune juice.