There is no room in that POS for a second JS. Well, unless both the Captain and the second JS'r are jockey's or...
It's his JS, not yours because you showed up and needed a ride. I did it once on SWA before I ever flew it, and I never will again. A buddy took the port seat, and my fat a$$ was jammed in the center. If you feel as though he was out of line, I don't understand your umbrage. He did you a favor.
I fly with the seat at the aft stops on every Boeing I've ever worked with the exception of the Triple, so I appreciate and fully understand his opinion.