my son got 120 in jee advance and we come under OBC what are the chance of getting the seat in IIT

Unlikely. It all depends on what your peers score. Look at previous year cut offs to generate probable rank to get an idea. Even if you do get in with a low score rould you be able to manage the rigorous course with a low score and inadequate understanding of subjects? The goal is to have a good grasp of your subjects.