Bob, I have some experience in this area since DS 6 always had a very large head and had to see a Ped. Neurologist at the same age as your nephew. I think the term they used was hydrocephaly?
In any case, he had an MRI at 9 months old and did not have excess fluid but the doctors did tell me that the procedure your nephew may be having is generally very successful and he should go on to live an absolutely normal life.
Has your nephew had any issues with seizures, dizzyness, disorientation or lack or balance?
My son's head is bigger than mine actually, and DH had to get him fitted for an adult sized bike helmet. His head circumference was always off the charts and it's now 23 inches around. I joke about it to him now and tell him he's got a lot of brains in his head, but I remember how we were always so nervous about his head being bigger and bigger at each Dr. visit and the MRI almost put me over the edge.
Hugs to your little nephew and your family.