Difference between 就要。。。了 and 要。。。了
In practice, I doubt if anyone really says 明天他要回来了 although there seems to be no grammar mistakes in the sentence. 明天他就要回来了 or 明天他就回来了 is more likely to be used colloquially.  

I think it probably because the way Chinese expresses this is different from English.  If we want just to express "he will be back tomorrow", we   would say 他明天回来.