So we met with New Doctor today. (She's okay, but she's no Great Doctor!)
Blood test from Tuesday showed his alkaline phosphatase level at 360. Normal for his age is anything under 480. Previously it had been 4081, and then 7100. This leads New Doctor to believe that he had a condition ("phenomenon") called Benign Transient Hyperphosphatasemia of Infancy and Early Childhood. Apparently in adults high levels of ALP virtually always means there's something seriously wrong, but they're only recently finding that sometimes, in young kids, levels can be ridiculously high for no reason at all and then level off on their own. They have a theory that it is a young child's overreaction to common viruses such as colds (or Norwalk, which we had 6 weeks before the first blood test was done). This is a condition/phenomenon that Great Doctor had told me about, but he (and Children's) also felt that it wasn't likely to be the case as he had so many other indicators of bone disease. But normal levels mean there's not.
Bottom line: no bone disease. Yay!
But that leaves us with more questions. He's still sinking like a stone through the growth charts, in body size. Less than two months ago he was between the 10th and 20th percentile in both height and weight. Now he's an even 10th for both. But during that time his head grew more than a quarter of a centimetre. He's still above 97th there. New Doctor comfirmed that this is of "great concern". Until about 10 months of age he was squarely on the 50th percentile for everything, and somewhere in the six months between then and the first time we saw Great Doctor something went crazy.
Brain ultrasound apparently showed nothing but came with a recommendation to get an MRI done. So that's where we are now. Referral to Children's orthopedics department was cancelled and replaced with a new referral to Children's neurology department.