qualifying dependent - full time high school student living at private school

My son lived away at a private prep school in 2018. He graduated that year and was under the age of 19 at the end of the year. He had no income and I paid all of his expenses both at school and while he was at home. He lived at school for a total of seven months. Am I able to claim him as a dependent for the 2018 tax year? I am thinking about filing an amended return.