"His thinking is outside the box."
Translation:החשיבה שלו היא מחוץ לקופסה.
Not idiomatic. Admittedly this idiom is absolutely burrowed from English... but it already has an accepted form in Hebrew and it is מחוץ לקופסה.
Interesting. Kinda like הנעליים גדולים עליו in place of "he has big shoes to fill."
To me, הנעליים גדולות עליו means literally, the shows are a size too big. The burrowed idiom would be something like: נעליים גדולות למלא.
your translation actually says "his thought is outside of the box", not "his thinking is outside of the box"....