C.S.
I would say the visiting toddler has marked his territory if you will. My son has a similair situaiton at preschool. Him and his BFF are fine and then lately the BFF has been pushing our sons buttons and it turns out as you describe.
I think the key though is matching punsihment. My guess is the visitor views himself as the leader because your son is the one removed from the situation so he's decided it is all HIS HIS HIS! :) I would try eliminating the naughty chair and next time it happens and the boy asserts himself try something like..ok, this is not ok-we have to share...you can each play with a car-pick one..if they want the same toy they take turns.