1) He stood up to show his brother that he wasn't paralyzed. It was a mistake on Dempsey's part to shoot him from that position.
2) He shot a hole in the wheelchair to strengthen his case for self-defense.
3) Dempsey saw Rhinehart pick up the letter opener and figured that he was going to stab him. He just gritted his teeth and took the pain. It was either that, or his faking of paralysis would be revealed.