"He is going to read seven books" would be translated as "Sta per leggere sette libri." While "Leggerà sette libri" would be translated as "he will read seven books". Correct? Because having intentions to do something is not the same as being sure you'll do something.
Native (Am) english speaker - Normally they are interchangeable, however in school many, many years ago, "going to" and "fixing to" (I'm from Texas, after all) were always considered a somewhat vulgar (unrefined) way of expressing the future tense.