>> The options are: cite him, slight him, or prove him wrong.
Again, are you a researcher? These are not the options. Also what even is "slight him" or "prove him wrong"? My comments above are about citation practices not this one researcher in particular.
>> I think he's made it clear that he won't accepts slights without a fight, so one can either show him where he's wrong, cite him as he plainly makes clear one should, or admit that he's being shut out for unscientific reasons.
There is nothing to fight about nor nothing to "prove". There are many reasons to cite an author and being the first to publish a paper many years ago about a topic does not automatically deem citation. That's not how science works. It's how (unfortunately) the academic churn of paper writing works at times.
FYI: it's not "unscientific" to not cite a paper. It's almost impossible to cite all relevant works, so the author of a paper selects the most relevant. Obviously this is biased and can become political (e.g., Schmidhuber), but flamming online about not being cited doesn't mean you must cite an author.