Based on the discovery requests, it appears Amazon is mostly requesting documents and discovery from him that CA can't help him with, since Molson is in control of those documents.
Being an expert witness usually means that the party employing your services generally pays for your time to respond to discovery requests. In this case, it appears he was cited in CA's complaint but CA did not actually engage him as an expert witness so it doesn't appear (based on the Twitter thread) that he is being paid for his testimony.
As all of the requested documents should be standard business records kept in the normal course of running his sales business, and he does not have any employees, "reasonable compensation" in this matter is presumed to be zero unless he can demonstrate to a judge otherwise.
Being an expert witness usually means that the party employing your services generally pays for your time to respond to discovery requests. In this case, it appears he was cited in CA's complaint but CA did not actually engage him as an expert witness so it doesn't appear (based on the Twitter thread) that he is being paid for his testimony.