Ask your nephew if he has had a fall, or some type of injury? Ask him carefully about this. My daughter, when she was 8, got a persistent headache. On the third day I took her to a doctor and explained that some time before she had been slightly injured. (I was to learn later that she had a couple more "minor" injuries after that.) The doctors kept looking for infection, even though I insisted it was an injury. The long and short of it was that she soon became a semi-invalid. I took her to several pediatric specialists, all of whom kept looking in the wrong direction. Four years later, getting no help from anyone, I took her to a chiropractor. Fortunately he was one of the really good ones. She was cured within three weeks, although there were some residual problems. She leads a normal life.
So I repeat, question your nephew closely about any injury he may have suffered.
Hopefully, by now you have talked with a doctor. This is not a typical anxiety attack! Almost sounds more like a concussion. What was he doing before the problem? and what is the reference to "painting"?