As Chris' father, I would have begged him to return, and (as his father did) make every effort to find him and make sure he was safe. A serious sit-down would be in order, though neither Chris nor his father seemed to want to take this step.
Of course, this is completely an opinion question, and hopefully gets moved to the discussion section of the site so you can get more suggestions. But what would you say? Everyone interprets literature differently, so don't be afraid to go off on your own no matter what I may think.
There were unresolved issues between McCandless and his parents, ones that were serious enough he broke off all contact with them. We get the impression reading the book that he has no intention of reclaiming his parents at any time in the near future. So this would affect any conversation I would have with Chris.
As a parent, I wouldn't try to talk him out of his Alaska adventure, as this would only make him more likely to try it. I would try to convince him to let me buy him some proper winter clothing and food so that he could survive it. I would be angry at him for being so careless and foolish with his life, tempting nature as he did, but with our strained relationship, I doubt I would have been able to convince him of much.
The only thing different I would do as his mother (as the bond is different between mother and child) is to convince him how much I love him. I would try to repair the relationship at least on that level of making him aware he is still loved no matter what, and that I am concerned for his safety.