The JEE MAINS 77.75 %-ile have a predictive overall rank of 227618 - 255875. Now to be eligible for the JEE ADVANCED the student have to clear the JEE MAINS cut off for 2021.

The expected cut off for JEE MAINS 2021 SC category is going to be 50-55 percentile. So your son had good chance to be eligible for JEE ADVANCED 2021.

To know more about JEE MAINS cut off you can visit the link provided below:

https://engineering.careers360.com/articles/jee-main-cutoff

To predict the colleges through JEE MAINS 2021 you can use the college predictor:

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

For JEE ADVANCED preparation strategies you may visit:

https://engineering.careers360.com/articles/jee-advanced-2021

I hope my answer helps. All the very best for your future endeavors!