IIT

Joint Entrance Examination (JEE) is conducted to offer admissions to interested candidates in undergraduate engineering programmes at NITs (National Institutes of Technology), IIITs (Indian Institutes of Information Technology) and other centrally funded technical institutions (CFTIs) located across the country. JEE Mains 2018 offline (pen-paper) exam will be held on April 8 while the online exam will be held on April 15 and 16. JEE Mains 2018 registrations have commenced from December 1 and concluded on January 1, 2018.
Regards.

IIT offers several applied and pure science courses, a few offer courses in Economics, Statistics and MBA too. Check on the course you are interested in and apply for admission procedure.

Hello Vasu,
Here are few best colleges for Aeronautical Engineering except IIT and NIT.
1. Sathyabama University, Chennai
2. Acharya Institute of Technology, Bangalore
3. MS Ramaiah University of Applied Sciences, Bangalore
4. Hindustan University, Kelambakkam
5. Malla Reddy College of Engineering and Technology, Hyderabad
6. Bharath University, Chennai
7. Faculty of Engineering and Technology, Manav Rachna International College
8. Kalasalingam University, Virudhunagar
9. Sastra University
10. Gujarat Technological University, Ahmedabad.

Both are great institutions. But, according to my point of view, IISC have a benefit for researchers.

Colleges accepting IIT JAM 2018 scores/ranks for admission are mentioned at the link provided below. Candidates are informed of the total number of seats available for different M.Sc courses offered at all participating institutes. A total of 15 IITs, 16 NITs, and two Centrally Funded Technical Institutes (CFTIs) accept JAM scores for M.Sc admissions:
https://www.shiksha.com/science/articles/colleges-accepting-iit-jam-scores-ranks-for-admission-blogId-13971

Yes, he can get NITs but not any of the IITs.

It is less likely.

You can get IIST if your JEE Mains rank is good. You have low chances for IISER.

It's your choice if you balance between your school study and JEE preparation then you can start preparing for it in class 10th.