It could be that his WBC has been dropping as his body wins the battle with the virus. Some of these viruses can hang around in the body, battling with the immune system, winning then losing then winning etc. Once the body's beaten the virus enough to reduce the symptoms, very often we aren't even aware that our bodies are still fighting, because we feel so much better.
Its a really good sign that he seems fine in himself and that all his other results came back normal.
I was convinced ds1 had something dire, as he had lumps in his neck like huge bunches of grapes for month, upon month, upon month, but he was fine in himself, his bloods were pretty much normal and that's why they weren't concerned about him. Had he been debilitated and lethargic or in pain they would have been much more concerned.
Conversely, ds2 has just spent a few weeks really debilitated and weak, congnitively fuzzy/confused and getting repeated instances of styes, tongue ulcers etc. His bloods came back as Hb marginally low, WBC normal, but the GP was more worried about him because he was so obviously poorly.
When I had pneumonia a few years ago, my WBC was apparently over triple the normal range, so it was immediately obvious there was a problem that needed dealing with, despite my symptoms not fitting. They took several more sets of bloods over the next couple of weeks to check my count was coming down and once it was approaching the normal range - although still slightly elevated - they didn't feel they needed to do anymore.
I'm probably not doing much to reassure you here, I know. I would be worrying just the same if it were one of mine, its only natural. Try not to torture yourself though, all the signs seem to be suggesting that there's nothing serious going on and at least everything is being thoroughly checked out, so you can at least be reassured by that.
Hope you get the answers you need to reassure you when you next speak to the GP or one of the other docs.