Since almost anything can mean almost anything in English, it would be a great help if you provided complete sentences when you ask questions. I am not even 100 percent certain what you mean by sentence. Anyway, the sentence in which he is not sure is perfectly all right in a sentence ([ ]) like this: The sentence in which he is not sure about his future is at the beginning of the book.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.