Computer science is a very lucrative career when it comes to technology. Law per say isn't that bad, but it's highly demanding, populated and time consuming. Besides, it's one of the oldest career ever.
My advise to a nephew, is to study computer science as a course, because as it's stand now, technology rules the world. Develop more on your programming skill if you want to be a guru in computer science.
I wouldn't choose anything for him. I'd let him choose whatever he wants.
My role would be to explain him in detail what to expect from these courses and later on in life and make sure he understands what he's getting into.
You can make a decision for him, you have to let him choose what he likes. Being forced to do something you don't like can have negative effects and can destroy you, especially at a young age.
First, where does his interest lies.
Having to choose between the two based on income, I will advise he choose computer science. The oppurtunities in that field is enormous, the only limit to how much he can make is his passion which is why I asked where his interest lies.
Learning computer science in college/the university is just to get him started. What he will become and make will all be about how passionate he is in that field and how much efforts he is willing to put in. Learning computer science in the university will not make him stand out from others.
If his interest lies in computer science and technology, tell him to go for computer science and put in hours not just to get good grades but to have an edge and become an expert in whatever aspect of Computer science he chooses.
As an expert computer scientist, he does not need to be fully employed in a company to earn meaningful income. Of course there will be no shortage of job offers if he is really good but he will like prefer to have some freedom. There will be abundance of freelance jobs.
Both courses can bring a good source of income depending on how he utilize them.
I will advise such a nephew to go for what his heart desires because at the end of the day there is Joy in doing what he loves. And I have this believe that successful people are the one that make a personal decision of what they like
Either one of these fields might generate if you are good at it. These two fields are extremely opposite, usually people are not be able to good at it both. Law generally requires emotional intelligence , being able to persuade people. For computer science generally being good at math and pysics is enough.