Originally Posted by CDTraveler

No, logic says you sit in the seat shown on your boarding pass. You don't know why or how he was assigned the middle set, and for all you know, maybe he likes the middle seat best.

If it really bothered you that much, you could have politely suggested he take the aisle seat so both of you would have more room, but if he wanted to sit in his assigned seat I don't see that you have any right to expect him to move to make you comfortable.