He's still grappling with maturity. It will click. Keep reminding him that the point of "taking responsibility" is not to "feel bad" it's to understand what to do next time. Sure, if he wants to take "boo boos while watching baby brother" as a sign to watch him closer, then Good Work! Pat on the back for recognizing the sign, and go forth and conquer. Not "Oh no, I did something wrong and it's all my fault (sad face)."
Help illustrate this until he gets it. The core of the principle is good though, knowing that on some level almost everything that happens to us is our own fault to some extent.....even not maintaining the car well before it breaks down sometimes....helps us to make wise decisions and choices going forward. But there is a lot to be said for realizing that sometimes sh__ happens, because thats' an undeniable truth for everyone in the world. Old Westerns are good for watching brave macho men take responsibility and be tough with a certain amount of valor and no victimy self pity. Rent some! Once he gets the gist, he'll be great!