It sounds like your Louis emigrated to Kenya sometime between 1891 and 1900. That is why he would not appear on the 1901 census.
When the Great War broke out he was already 43 so he was not likely to have returned to Britain to enrol in the Forces. There was some action of a naval type on Lake Tanganyika between British Colonial Forces and the Germans who actually had a armed steamer there that they had reassembled on site. You may look in good library for detals and find him mentioned.
The white population of Kenya was not great back then, a handful of people, but you will not find information easy to come by. If he is not a direct ancestor but only a blood relation of your husband then I suggest you put this on the back burner and get on with other ancestors.