May be that he is here, there and everywhere in his mind. Jupiter ruled people tend to be quite eclectic and therefore, more prone to distraction then perhaps a Capricorn might be, or a fixed sign might be like Scorpio. Less focused in general unless of course aided in the natal by other factors.
I would look towards the fact that Sagittarius is a mutable sign and ruled by Jupiter. Being Mutable might allow a bit of room for some loophole or absentminded ness. Being ruled by Jupiter might have their focus vast and expanded over a broad subject area.