[Orophin]

Theoden was pretty young when Thorongil aka Aragorn fought with his father. It is possible that he did not recognize him for a number of reasons. One may be that he simply doesn't remember him because it was so long ago. Two maybe he remembers the person but doesn't connect them because Aragorn lokks differnt from traveling in the wild so long. Third maybe he does know it and he just doesn't say anthing for who knows what reasons.