Is it possible for my son to renounce his PR status before getting enlisted to serve the NS?

Yes, it is possible. There are cases where people have renounced the PR status in order to avoid national service liability.

However, if PR from second-generation do not serve the NS and renounce their status, this will result in unfavorable consequences. For example, they will surely be barred from getting PR status in the future. In many cases, their application for Singapore Employment Pass will also get rejected.

However, there are still cases where former PRs become successful Employment Pass applicants.