The D6 visa is intended to allow family members to be reunited with the residence card holders in Portugal. That would be the visa you should apply for after your son obtains his resident card.
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What visa do I need to stay in Portugal while my son is attending high school there?
We are planning to send our son to a high school in Lisbon as an international student. I want to be able to stay in Portugal in order to take care of him. Which type of visa or resident permit should I apply for? What are the requirements?