I have to agree with BOAC on this one, it is up to you as the Captain to set the tone. I'll be the first to admit I have a strong personality (alpha bitch?) and I have noticed that when there is a lack of leadership in a group/situation I tend to assert myself this role, whether it is appropriate or not. Perhaps there is a bit this in play? But the laptop thing is clearly unacceptable, the pax are paying him to fly/monitor the aircraft, not tend to his personal business. (In dubious situations I would always ask myself "How would I explain this to the NTSB?" That generally provides clear path.) It is up to you as the Captain to hold him to standards.
Best of luck with this!