Basically - and this sounds odd about another NBA player - but he's not coordinated enough to have a reliable post move. I've seen him try jump hooks and up-and-unders and more often than not he messes up the footwork or doesn't have the touch to finish. I can tell he works on those kind of things, but it's beyond his physical abilities to do it well, consistently. I'm happy that he's at least learned how to establish good post position so he can get the ball and go right up with a dunk or lay-in. TBH, having watched him his whole career, DJ is way better than I ever thought he would be.
And that's really terrible news about Bosh.