Sujit, Delhi has an Institutional quota which is for Students who have completed their 10th and 12th from Delhi. It was previously 10th and 12th and now they changed it to 12th alone. So by the virtue of studying in Delhi, your sons becomes eligible for Delhi State quota. His 10th and 12th certificates will be enough to prove hie eligibility for Delhi State Quota. He doesn't need a domicile certificate in particular. But as a native of West Bengal, he should have West Bengal domcile certificate. This is because he has studied out of the State for considerably 4 years and so it is important for him to have West Bengal's domicile certificate.