I chose for my barbarian the Mordenkrad (2d6 Brutal 1).
If he attack with Howling Strike Hit: 1[W]+1d6+Strength,
can he reroll the extra 1d6 given from the power, if the roll is 1?
Or the reroll applies only to the 1[W]?
The only dice that are rerolled are the weapon dice. So what ever is listed by the [w] reference. For instance if you had a high crit brutal weapon you would also get to reroll that damage since it is listed as an extra [w]. No other dice are rerolled.