As @Rokaab suggests, your son needs to see a doctor for diagnosis of his problem, from what read metatarsalgia has several causes and only a doctor by taking a history, performing an examination and most likely ordering a test or three will ge able to sort this. Sometimes a sports physician may need to be involved as well.
Best Wishes for a quick solution!
What should you do? Absolutely, definitely seek medical attention.
He may be referred for more tests or he may be referred to a physio.
But an active 15 year old who gets pain when he walks is a problem the Internet cannot solve.