This movie did such a good job of showcasing why I love Downeys Stark so much. Yeah he comes across as a wisecracking jerk a lot, but his past few movies (even going back to his first, actually) have shown that he uses that to hide just how personally he takes it when harm comes to people. He feels he has the intellect and tools to save people, and when bad things happen to good people, he takes that has a failing on his part, so their pain is his fault. He takes everything so personally even when he isnt obvious about it (the kid in IM3, Peter in SM, the death of the boy in CV), and he worries about things so much hes developed PTSD from it (even if it doesnt haunt him as much as it did in IM3).
So in this, when he finds out about Thanos, he instantly takes it seriously and gets into the mode to figure out what has to be done. And by the end, when hes fighting Thanos with so much on the line, he is literally giving the fight every ounce of what he has.
What I really love about Downeys performance is how believable he is as a guy who will give you grief one minute, but literally fight to the bitter end to protect you the next.