I think that cowboy's screwed.
Seriously, though, this has a lot of energy- it's looking really good!
As for suggestions, something seems a little awkward about the T-Rex's position. I almost want to see his right (our left) leg pushing backward, away from us, to heighten the sense of movement.( You could do the same thing with the other leg, if you remove the obscuring dust clouds?) Right now, he seems to be crouching more than running.
Otherwise, I like this a bunch- the idea is great and the rendering is fantastic- especially for "just" a sketch! Can't wait to see the finished product!