Chicken pox usually start on the stomach area and then spread elsewhere. It's a mild routine childhood illness, and does not require a doctor's attention unless your child already has immune problems or gets really sick. My kids haven't been vaccinated, and my older son has already had the pox (my husband had an outbreak of shingles when my son was a year old, and that gave him and all of his cousins chicken pox). I'm hoping to expose my younger son before he gets too old.
I was 12 when I (and most of my friends at the time) got chicken pox, and I must say I wish I had gotten it when I was younger, although it's not fun at any age. Somebody said something about "bringing out the pox" -- well, I don't think you should do that. Believe me, if it's chicken pox, they'll come out; and why should you encourage more of those itchy scabs to break out than otherwise would? The worst cases I heard of were in the kids whose parents gave them oatmeal baths and other stuff to "draw the pox out". Well, it worked -- the kids were covered in chicken pox. One girl had pox in her vagina, another had several in her mouth.
If it is chicken pox, the spots will get worse and be all over his body. Just treat it like any other childhood illness, because he'll get a fever and feel pretty yucky for a few days. He'll be contagious for I think 2 weeks, if it is the chicken pox. You can look for pictures of chicken pox and pock marks online, and see if those spots on his back are pox, but more likely it's just bug bites or something.