Agreed. He essentially can't meet Djokovic or Nadal ever (in a slam), and he can't draw Federer on any surface but clay (now, that is; Federer would have easily beaten him on clay too in the past). He can beat Murray on clay, but his chances on the other surfaces against the Scot aren't great. And he can beat anyone else on the tour, especially on clay.
The draw for this Roland Garros is probably the best chance he'll ever get. He can't draw Djokovic until the semis at the earliest, and he can't draw Nadal until the final, so they'll be facing more quality opponents before they theoretically meet Ferrer. If Djokovic and Nadal go out relatively early (not unthinkable given their respective forms this year), his odds go up substantially.
It is unlikely in my opinion, but not impossible.
The big 3 seem far more beatable than last year, he will have his own quarter (because of Murray's withdrawal)... I don't know how he would do against Federer on clay, honestly, but Fed hasn't been very impressive lately.
He was 2 points away from beating Nadal and took him two sets for the first time in 5 years (I think?). Of course, this is provided that Nadal doesn't suddently play an out-of-this-world tennis because it's the FO, which might very well happen.
Nole would probably be a bigger problem, but then again, I never thought Nole would lose so often this year, so...
He needs some luck, and another thing he needs is to play as he has been playing lately, and not as he was playing earlier this season. Otherwise it won't even be Nadal or Djokovic taking him out, it might be anyone else, really.