Sir with this percentile he is only qualified to sit in Jee Advanced 2021. Based on his performance in Jee Advanced he will be getting IITs. The exam is not yet conducted. And the date for Jee Advanced 2021 is October 3 2021. I hope he has applied for the Jee Advanced 2021 as the application process in now over. And on 25th September the admit cards will be released.
For more details please refer the link for Jee Advanced 2021.
https://www.google.com/amp/s/engineering.careers360.com/articles/jee-advanced/amp

Also, Jee Mains only makes eligible to get NITs/IIITs/GFTIs and other colleges accepting Jee Mains score. It is not a gateway to get IITs.

I am also listing some colleges for him where he has chance of getting mechanical engineering.

• Netaji Subhas University of Technology
• DTU - Delhi Technological University
• Institute of Infrastructure Technology Research and
• Pondicherry Engineering College - PEC

For more predictions based on his rank in Jee Mains he can use the college predictor tool.
https://engineering.careers360.com/jee-main-college-predictor?utm_source=qna&utm_medium=jee_cp