It could mean he's just working or studying and doesn't want to be disturbed or he could be just taking a nap from all his studies to refresh himself.If you called him he might get distracted and end up thinking of you instead of his books !
What does it mean when a guy turns off his phone when you call him?
It could almost anything,but every time he turns it off hes avoiding you while cheating on the other person.
He probably wants alone time, and doesnt want to get disturbed. Or maybe his phone is dead, so he wants to shut it off, to not waste a lot of battery. Don't think he might've shut his phone off, because of you. It may be another reason.