Hello Sir,

It is difficult to predict JEE Main percentile because JEE Main percentile is depending upon too many factors. But according to you, your son scored 122 marks in JEE Mains 2021 (20 july 1st Shift) then your son percentile and rank  would be around 95.33 - 97.11 and 29565 - 53705.

You can predict rank from the link given below

OR

You can also check out Marks Vs Percentile Vs Ranks from the link given below :-

JEE Main 2021 Marks vs Percentile vs Rank – Calculate Marks using percentile score (careers360.com)

To more information about JEE Mains :-

JEE Main 2021 Exam - Answer Key (Released), Results, Paper Analysis, Question Papers (careers360.com)

Hope this information helps you...

Best of luck :)