15 is not too young at all to start thinking about this, as there will be academic hurdles to deal with on top of excelling in the sport. The NCAA will want to see detailed school reports from about the 9th year of school on.
I am in the US and I know students who have been scouted from age 12 on by college (i.e. 'university' coaches), several in softball, but also in tennis, track and wrestling.
Your student will need American SAT or ACT scores. These tests will require preparation. Look at collegeboard.org for information on preparing for these tests and test taking itself. You will need to set up an account in order to register for test taking and for scores to be processed. Prep for the tests will require a good deal of time and commitment. Start early.
www.ncaa.org/compliance/academic-guidelines
National Collegiate Athletic Association (NCAA) guidelines/ requirements.
pub.s1.exacttarget.com/5uvymtxo11g
Normal NCAA email to prospective student-athletes from the UK. Specific instructions are included in this email, as well as links, with a timetable, specific tasks and deadlines, etc.
Good luck! There are about 17,000 international student athletes in the US.
Be aware though, that if your student should end up admitted to an American university, he will only retain eligibility for the scholarship as long as he remains a student in good standing (there is a minimum grade point average - aka GPA) and as long as he continues to make the team - an injury could end his academic career in the US as well as ending his sport dream.
Be choosy about the universities on your list. Not all university degrees are worth the paper the diploma is printed on.
Lots of American students, both athletic hopefuls and non sports students get unsolicited offers in the mail from companies running what amounts to travel agent services for 'leadership experiences' or other experiences, and all you have to do is fork over several thousand dollars, for example, for a week in Washington DC, attending a few lectures, doing tours, and what not. The letters begin with a 'Congratulations, you have been chosen!' spiel.
So be careful - the 'you've been chosen' bit is sometimes about extracting money from you.
diycollegerankings.com/should-you-use-an-athletic-recruiting-service/3354/
Scroll down to 'Questions to ask an athletic recruiting service'.