Why was he wearing a 2 star general's helmut ...
even after he was promoted?
shareI don't think he was.
shareI don't remember it, but it would be ironic, after the early scene with General Bradley, where Patton puts another star on before the Senate has approved it. He tells Bradley, "They have their timetable and I have mine."
shareAt the start of the movie (giving his speech to the troops), Patton is wearing a helmet showing four stars (i.e. a four star general).
Shortly after in El Kouif, he is suddenly a two star general, promoting himself to three star (in front of Bradley) even though it has not been approved by the senate.
The speech at the start is later in his career- the rest of the movie is in flashback, so to speak.
Trust me. I know what I'm doing.
Patton's military career as a general was a mess: he was promoted to temporary Brigadier General (one star) on October 2, 1940. Temporary Major General (two stars) on April 4, 1941. Temporary Lieutenant General (thee stars) on March 12, 1943, but "relieved" of his command three days later because of a conflict with the British. He received retroactive promotions to Brigadier General (September 1, 1943) and to Major General (August 18-September 2, 1944. (Source: my own copy of Trevor N. Dupuy's The Harper Encyclopedia of Military Biography)
I am not sure when he was temporarily or definitively promoted to General (four star), but it's obvious that at the time of the opening speech he was one and wore four stars.
https://www.google.co.uk/imgres?imgurl=http://cdn.history.com/sites/2/2013/11/george-patton-H.jpeg&imgrefurl=http://www.history.com/topics/world-war-ii/george-smith-patton&h=454&w=1389&tbnid=IhfVxBKwZS2BTM:&docid=LDW8PSks4iBaBM&ei=bhSKVuW9G4r6aorThegG&tbm=isch&ved=0ahUKEwjlrInyxo_KAhUKvRoKHYppAW0QMwhgKCIwIg