When Prince Charles becomes King, Prince Louis' title will change, I think, to HRH Prince Louis of Cornwall and Cambridge (just as King George VI was briefly HRH Prince Albert of Cornwall and York between January and November 1901). I assume that, according to tradition, Prince William will be created Prince of Wales shortly thereafter though. Then Louis' title will change to HRH Prince Louis of Wales. When Prince William is King, however, if Prince Louis is not given any peerage, he may become simply HRH The Prince Louis, in which case his wife would be known as HRH The Princess Louis under current conventions.