Hello Aspirant,

Unfortunately, he cannot use his EWS Certificate now if he had not filled his category as EWS in the application form of JEE-Main because the admissions will be made on the rank mentioned in your scorecard and if there is EWS rank then your cousin can use his certificate in Counselling.

For complete details of JOSAA Counselling refer to the link given below :-

https://www.google.com/amp/s/engineering.careers360.com/articles/josaa-counselling/amp

You can also use JEE-Main College Predictor because it might help you in your counselling :-

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

I hope this information helps you.

Good Luck!!