Not with my child, but with me. I had a very low neutrophil count at one point. It turns out that this is a common effect of many viruses. My count stayed low for about 2 weeks after the virus symptoms went away, and it's been fine ever since (that was years ago).
In the meantime, during those 2 weeks when the count was low, I had about a million other tests because my doctor wanted to make sure it wasn't something more serious, because a low WBC count can be an indicator of many many things. I also had blood draws every other day monitoring the count. So don't be surprised if your doctor takes this very seriously just to make sure your son is ok and it really is just because of his recent virus-caused illnesses.