I do not have much experience in writing an introducing letter; however I think that the following advice might help you. At first, you should tell him st about you who you are, where are you living and what are you doing (your job), you can tell him a little bit about your family, because the parents are very worry if their daughter date with a "bad" boy ( I don't mean you). En then, you can tell st about you and his daughter, how do feel about her and also your plan in the future. You also should tell him what you have known about him, it might make him feel sympathy with you. Maybe you don't like these suggestions very much, so just read it as a reference.

Best