Mr. Witter is certainly trying to do the right thing. At the beginning of the book, though, he and Cullen don't really get along. And the thing is, Cullen's not even quite sure why:
Here's the thing about my father and me. We got along just fine. Everything had been great between us since he had stopped drinking when I was thirteen. Then one day when I was around sixteen I decided to start being a bastard to him. I had no real explanation for it. Still don't. (3.49)
Since religion is most definitely on the table in this book (be sure to poke around this section to see what we're talking about), Mr. Witter's decision to quit drinking might be understood as him getting on the path to redemption. Importantly, though, this doesn't really work with Cullen. Cullen isn't having it, judging his dad negatively despite his efforts to get on the straight and narrow. It's not exactly a favorable representation of where the path to redemption leads.
But Mr. Witter keeps on trying. He wants to show interest in his boys' lives; when Gabriel goes missing, he starts checking-in with Cullen regularly about his interests and where he wants to apply for college. And he takes on Gabriel's search with single-minded determination. He stops going to work and just looks for Gabriel full-time, hoping that he'll be the one to bring his son home. So maybe the point is that the path to redemption is hard work, that when the going gets tough, the tough get going.