I'm going to respond to this just coming at it from my own personal viewpoint-- what I'd probably do in this situation, not that it's what you should do in this situation. So it's nothing more than a different way of looking at the game and the scenario you have established.
Personally... I'd agree with jadrax. I now see Background as the second phase of character creation... falling in between the choice of Race and the choice of Class. So if I were to create this character mechanically using the story hook you have chosen... first thing I'd do would be to select Race. Probably Human. Next... I'd have to look at the backstory and figure out just how much time I had spent in the barbarian tribe, and how much "adventurer function" would I have done that would warrant taking a level of Barbarian? For I now see Class as what I do as an Adventurer. Anything that has to do with character backstory prior to going out on the Road To Adventure, I personally would attribute to my character's Background.
Looking at the backstory... I see little in it to suggest that I did spend actual real time adventuring as a Barbarian (Class). Especially due to the fact that at 1st level, the Barbarian (Class) gains the Rage ability. And there is very little here that would say to me that this character should have any Rage functionality. To me... the barbarian-esque references are almost all about the character growing up, discovering he actually is not really meant for the tribe, and instead going out on the Road To Adventure as something else (Wizard).
So based upon this... I would attribute the entirety of his time in his tribe to his Background. Probably taking something like Guide. And then... once on the Road To Adventure, I'd go ahead and take a straight 3 levels of Wizard. Because that was the real turning point for this character... leaving his tribe behind and learning the ways of magic. So using that first level for Barbarian seems off to me. Especially considering that had this character been played starting at 1st level... would being a Barbarian really fit? Would you really start Adventuring as a Barbarian, then when you're out and about with your adventuring party, suddenly drop it and begin studying wizardry? That doesn't feel right to me, personally. It feels like I should already be using magic once having joined up with the adventuring party I got together with.
So that's what I'd do. Take Human for Race, Guide for Background, and Wizard for Class... and if I really felt the desire to bring more of my barbarian-ishness out... select spells known that are closer to feeling barbarian-ish-- Gust of Wind, Longstrider, Cause Fear. Stuff like that. Then as I level, move from more of the physical, potentially barbarian-ish spells to ones that perhaps are more universally wizardly.
That's how I'd probably do it at any rate.