First, I'm sorry you're having such a hard time. That is a tough position to be in. Unfortunately, sometimes, giftedness doesn't present itself in ways others expect (argh!).
Did something happen during the test (boredom, distraction in class...)? I frustrated my parents once when I became bored during an ITBS math exam and started filling in bubbles to create artistic patterns. DC19 had a difficult time sustaining concentration on parts of standardized exams and did better on conceptual questions than on computation, spelling... Just some thoughts.
Tutoring/finding a mentor in his area(s) of interest may also help (maybe less problems with home schooling if someone else can answer his questions?), as it seems you are trying out now. I thoroughly enjoyed mentorships around that age and grew quite a bit in the sciences and creative writing/foreign language when I had someone in the field helping and critiquing me. It helped somewhat with home schooling and helped quite a bit while I was in school.
