Geeez. Put it at
71° and make an agreement between the two of you that you'll not touch the thermostat for one month. Then see if how it reflects on the bills.
As far as being able to tell the difference between 70 and 72...you better believe it. Different people just have different levels of sensitivity. I'd bet you can tell the difference between 70 & 68.
If it's a little too warm for you, you
should close your vent. Believe me, you're better off having him use the main heater as opposed to one of those little "personal" heaters that usually rate 1500w. Those things cost a fortune.
Does he have a computer? Tell him to reverse the fans so the warm air is blowing out the front and over his hands.
Buy him some big fluffy slippers and some thermal underwear.
Or if it's feasible, seriously consider swapping upstairs/downstaris rooms with him...if you're too hot and he's too cold then that would be a good solution if the rooms are similar.