If "dreamer" does not describe Alan, then there is no word that does describe Alan.

Hi Augustin, In this case, and in all others like it, "nothing if not" means that he was a dreamer foremost. It's like saying, "If he's not a dreamer, he's nothing else." it indicates that he is such a dreamer, and dreams so much, that being a dreamer defines who is to such a degree that it is the essence of what his entire being. So, if he's not his essence, he is nothing.

If not = sino