The interview makes it pretty clear that it isn't being published against his wishes. He wanted it out there, just not while he was alive.

Now perhaps Salinger's son (and his widow) could be lying in the hope of profiting off his work, but if so he's chosen a strange way to go about it, blocking publication of his juvenilia and neglecting his own acting and producing career to spend what will apparently be the best part of twenty years collecting and editing it all.

Personally though, I think Matt Salinger's genuine. In fact, he comes across as extraordinarily respectful of his father's legacy. Would that we all had people willing to expend so much effort and care to honour us after we have passed.