I always get so freaked out about blood stuff and jump to the worst outcome possible. So please reassure that my doctor is correct;)!!
The short answer - 8 is fine. It is totally normal for white blood cells to be on the lower end of the normal range when the body isn't fighting an infection - the count will increase as needed.
The more complex answer is that the total WBC count is less useful than knowing how many there are of the different types of white blood cells. You can be immunocompromised with a WBC count in the normal (or high) range if you have too few of one type and higher levels of another. However, unless you have specific reason to believe he's immunocompromised, there's no reason to think that the doctor is wrong (the count probably also included the percentages of the various types of white blood cells, so the doctor was probably including that info in his thinking).
DS born 6/03, DD1 born 9/06, DD2 born 10/10, DD3 born 4/14.