if he has spots just now then he will stop being contagious in the next 5-7 days so the baby wouldnt be exposed to it. If you have had it already then that is good too as you should be immune and again wouldnt be passing anything on to the baby either.
I'm 33 weeks and ds2 has just got over CP. I've had it before so wasn't worried (as when I was childminding a mindee had it so I checked with GP and was happy - this was in my first trimester and baby has been fine). It's more the c/s date that would concern me but like the last poster said, you should be fine by then. My ds2 had them all crusted over 7 days after the first spot appeared. I'd just make sure you check the spots carefully before going in next week and make sure they are all crusted over but I'm sure they will be Best of luck x