I don't know if Tarantino is necessarily subverting it or not, but that character is way too fleshed out and unique to be considered any kind of a stock character.
As to a few points made earlier in the thread, the term I've always heard used for the white person who helps out the group of minorities is the "White Savior"*. See urban school/sports dramas: Dangerous Minds, Freedom Writers, The Blind Side, Glory Road, Hardball, Finding Forrester**, etc; historical dramas, such as almost every movie Zwick has made***, Lawrence of Arabia, Amistad, Last of The Mohicans, Avatar, etc.; and other films where the white person steps in on behalf of the troubled minorities: Grand Torino, To Kill A Mockingbird, The Help, The Ghosts of Mississippi, Cool Runnings...
*The first point I'd like to make is that not all of these movies have pat prejudices or themes. And a few of the ones I listed are even classics. But the central conflicts still resolve around issues of people of different ethnicities, and the main protagonists are all white.
**In a case like this, the issue isn't so much that Sean Connery's character is a mentor to a black student that makes it a white savior movie, but that he's a mentor to black student from the hood. Things like 'You the man now, dawg' are simply insulting, and this is one of the main problems with portrayals of minorities in film, the assumption that all of us come from impoverished, violent communities on the low end of the socio-economic ladder (and of course when there is a black or brown character that doesn't come from that type of community, they are generally portrayed as Uncle Toms).
***I recognize that some of Zwicks films, as well as other ones on here are based on true stories. But they still choose to cast the main players in the events as the white ones, and the fact is that (moving on to a different director), true story or no, a movie like Glory Road has no reason to get made. It's been done to death. At the very least, use one of the minority characters as the main character, and use their perception of how race affects the whole thing, rather than focusing on the good hearted white people standing up for them. Remember The Titans is a crap film all around, but at least it handles its racial themes in a dignified manner.