Ashya King and his parents, Brett King, 51, and Naghemeh King, 45, and six siblings, were found in Malaga, Spain, three days after the boy was removed from Southampton General Hospital.
The boy was being treated for a brain tumor, which had recently been removed surgically.
Brett King posted a YouTube video Sunday explaining why he and his wife decided to remove their son from the hospital.
He said doctors at the hospital advised using a combination of chemotherapy and radiation to treat Ashya, but Brett King wanted to take his son to a hospital in one of several countries that use proton beam therapy instead.
He said research outside the United Kingdom shows proton beam therapy can target the tumor directly, instead of pelting Ashya's brain with radiation. Brett King said doctors at the Southampton hospital said the proton beam therapy would not be useful in Ashya's case and threatened to issue an order of protection, preventing the Kings from being with Ashya if they questioned treatment again.
A spokesperson for the hospital said the Kings were offered the chance for a second opinion.
"Our priority has always been Ashya's welfare and we are delighted that he has been found," the spokesperson said. "Throughout Ashya's admission we have had conversations about the treatment options available to him and we had offered the family access to a second opinion, as well as assistance with organizing treatment abroad."
The Kings were arrested Sunday and Ashya was taken to a hospital in Malaga. The Kings were expected to appear in court Monday in Madrid.