Courses in these topics, should he decide to pursue a geology major, would give him a good foundation in the discipline (and may be required). He should begin by taking one or more as soon as possible, even if his interest is tentative at this point. Some minor adjustments may be possible depending on his interests:
Hydrogeology
Mineralogy
Petrology
Sedimentology
Structural geology
Paleontology
Field techniques
GIS