Unless I am mistaken in my reading-between-the-lines, he is upset about the volume of discovery in the State of California lawsuit against Amazon where he has been named an expert witness.
The volume of discovery would be completely normal in such a case, but would be rejected by any sane judge if Amazon were suing him personally for ... libel? I'm not sure what Amazon even could sue him for, criticizing the company isn't a crime.
Is the state not reimbursing/covering these costs related to their lawsuit? I can't imagine anyone would ever be an expert witness if they had to bear all costs either party or the judge imposes on them.
I don't know the answer to your question, but I do know that expert witnesses often charge extremely high rates for their services. Perhaps those rates are so high, in part, to compensate for all of this hassle.
I'm not familiar with how lawsuit process works in the US, but wouldn't the fact that he got served imply that he actually got sued by Amazon? Or is it normal that witness gets served as well?
Discovery is weird, and if you ever get a request for it from someone with more money/lawyers than sense, you really should get your OWN lawyer to push back, as there are ways to get them to pay for it.
It looks like Amazon is asking for documents (via subpoena) beyond what the state was in an attempt to overburden him so he decides to stop being a witness.
1. You may be able to file an Anti-SLAPP motion.
2. You could consider filing a complaint with the SEC.