I should add, that there are tons of people who want to be marine biologists, but there are relatively few jobs available. Your son should commit to at least a Masters degree, and probably have PhD in mind as well.
That said, it might not be a bad approach to aim for something like a Bachelors of Environmental Engineering first, with plans to immediately move into the Masters in Marine Biology program. That way, he can have something to fall back on. Just have him make sure he talks to the counselor and gets the pre-requisites for marine biology out of the way while he's at it.
Both our Marine Biologist (CNMI DEQ) and the Marine Biologist for American Samoa EPA took that route. For one of them, at least, it was the engineering degree that got his foot in the environmental agency door, and then he went off and got the marine biology degree later. His job was almost guaranteed because he knew everyone by then and ahd a good reputation. On the other hand, our marine biologist went into grad school right away (University of Guam for his MS) and never used his engineering degree.
And that brings up another bit of advice concerning the possibilities of actually getting a job. It might be best to go to a school in a somewhat remote area (Hawaii at a minimum, University of Guam or Fiji at worst) because there's less competition for jobs, and the agency people in the region will be more familiar with the subjects of your thesis, possibly even know you, and that might give you an edge over some Duke or Scripps grad. That's how it has worked out here, anyway.
It's definitely a tough field to get into, but if he can, the jobs can be very rewarding. Our marine biologist gets to tag along on the yearly NOAA research voyages to our uninhabited northern islands, and he's out diving locally every week. He spends something like 15% of our entire agency budget on boats and gadgets. I'm somewhat jealous, except that I realize I actually perform a useful service.... as far as I'm concerned, he and the other marine biologists are just collecting data, which no one is using right now... :jerkit: