It may not be chicken pox, actually I would be really surprised if it was. I know that you can still get chicken pox if you were vaccinated for them but if you child was vaccinated in the US then chances are he was vaccinated for varicella (chicken pox). And even if he did get the chicken pox the virus would be VERY mild. There are many, many other childhood virus's that can cause a rash and be itchy. I would take him to the doctor and have it diagnosed before you decide that it is chicken pox. Especially if he was vaccinated because they will want to know if he does have it to report that has it to infection disease for statistic purposes. 2 of my sons right now have a rash from head to toe and are itchy but don't have chicken pox, it is another virus. Chicken pox are blistery little pustules that come up in crops all over the body usually starting on the trunk and spreading outward. They last about a week and once they have crusted over they are no longer contagious. As long as they are getting new croppings they are still contagious. They problem with chicken pox is that they are contagious before they get the spots. They develop flu like symptoms, fever, sore throat, runny nose,loss of appetite, fatigue... Rarely it can be serious. Try to prevent scratching to avoid skin infections and scarring. Calmine lotion helps, so does oatmeal bathes.
Your best advice is to take him to the doctor to get a confirmed diagnosis.
Mom of 4 boys.