I have a lot of experience tutoring math in small classes to many students, including a couple who went on to earn gold medals in International Physics and Chemistry Olympiads. (I no longer teach due to my current research and startup work.)

I'd say your child would gain much more from having good private tutors you hand-pick from local schools/universities/private tutoring centers (depending on the level of your son's attainment and the qualifications of the teachers). Also, for certain subjects, you could be a great teacher yourself.

Even without taking pollution impact into account, using the hours saved from commuting in a very congested city to study one-on-one or in a small group would likely net a bigger gain than what quality difference there is between the good local school and a top school in the capital city. The key conditions are 1) having qualified tutors who fit with the student's personality and interests and 2) regular schedule so the student does not spend the extra hours playing mobile games instead.

Another key advantage is your child can use the best educational materials available on the Web, paid and free, such as Khan Academy, 3Blue1Brown, and the likes for learning. The presentations there are often head-and-shoulders above the techniques most school teachers typically use to explain concepts in class.

Edit: For a downvoter who I assume might disagree with the last paragraph, you need to go visit average schools in a developing country to see how the teaching is like.