Would he need a valid class one medical in order to get the JAA CPL?
Yes, a Class 1 medical is required for issue (or re-issue) of any professional licence.
And to confirm, he will be able to renew the CAA ATPL indefinitely and hold this concurrently with a JAA CPL?
Probably, that is how it was pre-JAA, but the UK CAA have not yet published their intentions for the re-incarnated national ATPL.
Finally, would the CAA ATPL retain the 'Grandfather' IMC rating
It should, but see the previous answer.
The fact is that the UK CAA is currently acting like a rabbit in the headlights. If you put the same question to Gatwick and Cologne, you get two entirely different answers. If the high-priced help at the Belgrano can't understand what's happening, what chance have the rest of us got?