Scratch the blisters on his leg (resulting in scabs) and touch his forearm with the oils from the blisters still fresh on his hands. It worked for me as a kid!
The rash does not spread by exposure from the blisters or from the fluid in the blisters. Common myth. It sometimes appears that way because different areas of the skin react slower than others. Another myth is that you become less sensitive with subsequent exposure, the opposite is true. Many do not react with the first exposure, or react mildly. I've had it severely enough to require steroid injections and now I'm hypersensitive to exposure.
There is a wonderful product “Zanfel” that is a soapy substance that removes the oil from the skin and absolutely stops the itching. It's expensive, but a little goes a long way.
The danger of staph or other skin infections being introduced by scratching is a possibility, so a doctor visit would be wise.