Since the Boer War was fought in South Africa, 1899-1902 you have a time frame for your search.  Since I doubt there was British Army recruitment in Lithuania, I would suppose that he immigrated to Britain before that.  You don't give grandmother's birth date, or the brother's...or what his name was in Lithuania.  With 20 years between siblings, there might have been siblings in between who passed down more complete information.  My advice would be to write out a more complete synopsis of what you know, with complete names and dates as you know them...even off-line, as if you were writing a story and you might answer some of your own questions and at least in the end you might better define what your question is.