according with the place where it is put "он полетит первым" one has the right to translate "he should fly the first"
That's right.
but the mention to предупредить космонавта заранее indicates that it refers to сообщили.
It doesn't exactly refer to первым.
The second sentence explains why he was told that he the one who will fly only a few days before the start (not in a more long period of time).