The most straightforward path for Trump is to hold all the Romney states (which looks precarious at the moment), and then win Florida, Ohio, and Pennsylvania.
Click the map to create your own at Even with Hillary at what may be at high tide in the post-convention period, Florida (Trump down 6 and 1 in the August polling) and Ohio (down 4 or 5 in the August polling) don’t look completely out of reach. The problem is Pennsylvania, where the recent polling has Hillary up about 10. She has room to drop and still have a comfortable lead in the state.
How does Trump get there without Pennsylvania? Again, if he holds the Romney states, wins Florida and Ohio (and assuming Wisconsin and Michigan aren’t happening), it’s possible to get to a 269-269 tie with states where he has been competitive, New Hampshire, Iowa, and Nevada (granted, this requires disregarding the recent New Hampshire poll that had Hillary up 17!).
Even on optimistic assumptions, there is no room for error.
Donald Trump & Hillary Clinton -- Electoral Paths to Victory