Oh god my ds had chicken pox on top of ezcema when he was 5 months. It was horrific, he looked like a shrapnel victim and ended up in hospital.
first thing - STOP USING ANY HYDROCORT if you are using it for his ezcema. It acts to suppress the immune system locally, and if he gets chicken pox when you are using it he won't have any resistance to the pox and will be COVERED in poxes. That is one reason why kids with ezcema get chickenpox badly.
Second - dose him with piriton at the first sign of spots, to relieve the itching. If he does too much scratching he will aggravate the ezcema but also infect it - and infected poxy ezcema skin is awful.
I'm not sure that Calamine does much for chicken pox, I think it is more of a distraction technique - and it is VERY drying so I wouldn't use it on an ezcema child. I would instead go to boots and get the Aveeno cream with Menthol, which is very emollient, excellent for extremely sensitive ezcema skin, and the menthol helps the itchyness.
In fact if I were in your shoes I would stop the hydro cort and go on a big Aveeno menthol moisturising bender in anticipation, to try and get the skin in as good state as possible, and then start Piriton and continue Aveeno at the first sign of spots.
Best of luck!!