This what I have been saying. And by being armed he could be considered the provoker. Look at 2a above directly from the WI law. He was engaged in unlawful conduct by open carrying a long gun and being 17. Not saying it is right but it is a fact.
He did try to retreat but did he do enough. Only a jury can decide that or he might be wise in getting a judge trial and just looking at the law not the feelings of jurors.
If carrying a rifle was unlawful, the kid lost any Castle Doctrine presumptions in 939.48(1m)(ar) and (b) --- not that the Castle Doctrine was relevant as he was out on a public street and not defending himself in a dwelling, vehicle, or business.
939.48(2) only deals with whether provocation requires a duty to retreat. Even then, a person can legally use deadly force when they have "exhausted every other reasonable means to escape" which would be true of a person on their back on the ground and being attacked by multiple assailants.
939.48(2)(b) removes even that duty to retreat after a person "withdraws from the fight" which courts all across the country have consistently ruled to include running away from a fight.