Why not "He needs to know all"? все has been translated as "all" in other lessons.
Йому треба знати все! He must know everything - its wrong! correct: He has to know everything. why??
I usually think of dative pronouns and треба literally meaning "for this person it is necessary to...", so this sentence could be literally translated "For him it is necessary to know everything." In English the sentence "he must know everything" could have the same meaning as the above, but it could also suggest that you assume someone is particularly knowledgeable, not necessarily that he needs to be.