Say what you will about the rest of the movie, but when Jimmy says that "Corey, I don't want to quit", it shows that Jimmy is still dedicated to doing what he has decided to do. It Makes Sense in Context.
The entire plot of the movie is so that Jimmy can make it to the Cabazon Dinosaurs in California so that he can leave a photo of his dead sister at the exhibit in the Stegosaurus that they loved.
The ending also shows where Jimmy's heart truly lies when it concerns family. Without any hesitation, he goes to hug his birth father, Sam.