I am on a TN visa and my son is studying in the university with TD visa/I20. I intend to change my current job and obtain a new TN visa soon. Can my son continue to study in the university on I20 alone if I don't apply for his new TD visa? I believe, Canadian citizen do not need a visa to study in the US but only require an approved I20 and pay the SEVIS fees. He does not have any plan to go outside of US during the course of his study. Thanks