Who needs a complete sentence?

As an educator, it's hard for me to meet students that do not see that education is important. I understand that college is not for everyone and there are ways of surviving without it, but the comments where students don't even see the point of high school are hard for me to really swallow. Today, I had a young man go on to tell me that he doesn't plan to even graduate and plans on dropping out of school soon. The hard part of this for me was that he states this to me as I am trying to explain to him the definition of a complete sentence. The student could not even understand the concept of a complete sentence and as I spent ten minutes trying to teach it to him he insisted that it was useless knowledge/information. Does he have a goal you may ask, like I did....Yes, it is to get his GED and become a tattoo artist. This is not the worst idea, at least he has a goal, but I asked him how he planned on getting his GED when he was struggling with the concept of a complete sentence. He said he was a fast learner...after fifteen minutes of trying to explain a COMPLETE SENTENCE, the young man insisted he was a fast learner and went back to his seat not knowing the meaning of a complete sentence. How does this happen? How does a student think that he can learn on his own when he is one of the students that I have to sit with and walk through step by step? The ignorance amazes me and at the same time I feel like our educational system has failed because why is a student who doesn't know the meaning of a complete sentence graduate to the ninth grade, which he did correct me and let me know he is actually a sophomore and my class is his only freshman class. I felt like this only made the situation worse and I was truly lost for words and a way to help him see the severity of his situation. Even now...I see him failing and don't know how I can really help him...