L.W.
Both of my sons had this as babies - it just means that they have been exposed to some sort of infection and have mounted some sort of response to it. My pediatrician advised that it can take months for it to "go down" and to just make sure it doesn't keep getting bigger. If it is somewhat moveable under the skin and remains the same or smaller as time goes on, sounds like nothing to worry about. Get the blood work done (they will probably look at his white cell count) for piece of mind, but this will turn out to be nothing, I am sure. L.