Here is a thread on schools with a strength in water resources:
Are you in one of these state universities?
See if he can shadow a professional civil engineer (or do some informational interviews), ideally those working in water resources/hydrology.
One useful skill he can start in high school is learning to use GIS (specifically QGIS which is free an open source).
When the time comes to decide which universities to apply to (which is not now), I would have him spend time on the course catalogs to determine the degree to which the university offers courses/specializations/research opportunities in water resources/hydrology/etc.
I see USF gives a full ride for national merit finalists and is strong in environmental/water resources: Environmental and Water Resources Engineering Engineering | College of Engineering | University of South Florida ; https://waterinstitute.usf.edu/
And so is Alabama: https://awi.ua.edu/; https://afford.ua.edu/scholarships/national-merit/