Beth,
actually, I did not pursue seismology; all my degrees are in geology. I just ended up using geophsyical data, and work on faults and folds.

I would pursue geology if I had to do it again. Not sure about other forks in the road: HW was offered a French government research position on a large tropical island...I said I would not be able to go because there would be no position for me.

Your son may be able to get into grad school but his options may be less. It will matter how he does on GRE exams and what his recommendations are. If he is unreliable/does not take school seriously, that could be a problem. I got my act together my last 2 years so I could bring my average up a bit, and get a good recommendation.

Chris