NDA

Not yet. He may have to wait another year to be eligible. 17 years is the minimum age required.

Between the two, it would be ICSE. But, CBSE board is truly a good option, if you are seeking to attempt exams held by Centre. CBSE books (NCERT) form the basis for such exams and NDA exam paper is made from these books.

Yes, but you will have to attempt Maths paper in entrance exam and pass in that. You will only be eligible for Army Wing however.

Usually, it takes 4-5 months, and for the January form, you will get it by mid June.

Well, you are not eligible for NDA, the option left with you is CDS examination for which you need to do graduation. And after clearing that, you need to undergo training in AFA Hyedarabad. After which you become a pilot in Indian Air Force.

Medical examination is the last examination for selection in NDA and it includes parameters, which are complex and understood by doctors and specialists. Written test - SSB (5DAYS) - Medical test. So you can apply for the written test and then can appear for irrespective of your surgery. But in the end it all depends on the medical.

These subjects are under the General Ability Test (GAT).
You can get the previous year's papers on Shiksha's link below
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.shiksha.com/exams/amp/nda/question-papers&ved=2ahUKEwjQz9LFk-7gAhVLRo8KHdQaBm0QFjADegQIAxAB&usg=AOvVaw03ak-lq0rSN2jqs0O92jEq&cf=1.

I will suggest you to visit the official website as there, they have mentioned the range in which they will allow you for the process.
All the best.

There won't be any problem. To know the teeth condition that can disqualify you are mentioned on this website https://www.ssbcrack.com/2012/10/what-are-14-dental-points.html.

Girls are not eligible for NDA.