Is he on the same family plan with you or does he have a separate plan? If he is on the same plan, you can look up his call records online. It would be just numbers without names, but still, you can see trends. If he has his own plan... I do not know, I think if I were someone who tried to conceal his call records, I would use codenames instead of real names in contacts and I would delete each call from the log instantly. I do not think that using a lock is the most effective way to be discreet about one's calling.