If you have received the mail that your son has been shortlisted for GD & PI then you don't need to apply again for anything. The only thing you need to do is follow the steps mentioned in the mail. If there is a questionnaire that you need to fill then fill it up. Questionaire might be in form of link or in a word document. Just make sure you fill it to submit it and take print out. Also, make a file of all the original documents along with their photocopy.

The whole process what you need to do, where and when will this second round of interview and GD will be happening for your son will be mentioned in a mail which you must have already received or will be receiving on the official mail id which you filled in your CAT form.

And one of the most important things is to ask your son to start preparing very hard for the second round because I don't know your son's overall percentile and rest of the profile so I don't know how much is the probability of him converting the call. There is no surety of conversion until and unless you perform very well in the second round because after this final merit list will be made to give admissions to the students which are based on a new aggregate score consisting of the interview and WSAT score as the highest weightage along with CAT marks.

So ask him to prepare hard and wish him all the best!!