My DS15 gets Anxiety like this with writing, he can write well but he sometimes just full on freezes and writes nothing. I feel for you, it's so frustrating and hard to work with.
Since it's possible he HAS missed something by skipping Math 6, have you considered finding a math program like ALEKS (
http://www.aleks.com/) and having him work through at his own pace say their Algebra readiness program. I'm suggesting ALEKS because it assess for where a a child has gaps and teaches to those gaps. There might be other online programs like this out there that might be better. I don't know how much aleks has changed over the past year when it was bought by McGrawHill.
Other than that for the anxiety, I have taken my son to a psychologist. I did this first in 6th grade and that initially helped a lot, he taught my son a lot of ways to handle his stress. Now I take my son to a psychologist that specializes in gifted kids and I believe it's been helpful. She has been helping him with his anxiety and depression, as well as his social skills.
