Hey Devendra.

See according to the JEE rules u should be getting ur rank under 240000 in the JEE MAINS exam to get qualified for applying the JEE ADVANCED paper. The other required criteria for that is the boards score should be greater than 75% for GEN students and 65% for SC/ST. If ur son stands on these two criteria them he is eligilbe for the JEE ADVANCED exam.

Hope u got ur point. Ask if any query.

Good luck