Of course you are welcome to give him the benifit of doubt and try to engage with him in good faith. I sincerely doubt it will lead anywhere, many have tried before you with no productive results.
Normally, you would try to falsify the existing theory, but, that is not an option, since it never existed - the proponents came with the claim that it had always been known and insisted relentlessly. It has subtle, nondescript symptoms, the only clear sign of the poisoning is its presence in the blood. There are some other measurable chamges, but that would only get stuck on me claiming that it proves its essentiality, vs you claiming that it proves the poisoning. All the evidence like the presence in bones, 250kyo neanderthal teeth, phosphates, coal and so on has supposedly been somehow proven irrelevant. The lack of improved health and intellect, rather the increasingly undeniable presence of the inverse is being ignored. It's essentially unfalsifiable.
> Two beliefs became entrenched:
1. that lead is natural to the human body, and
2. that a poisoning threshold for lead existed
Robert Kehoe, working for GM, was the chief advocate for leaded gasoline, and really the only person/lab doing research on lead until Clair Patterson stumbled into it while measuring the age of the earth. [0,1]
A modern equivalent might be if Facebook was the only organization researching social media's impact on society, while being able to set the paradigm/assumptions about said safety for half a century.
So even when Patterson's research was published in 1965, it took time to change the paradigm, and more time to phase out lead's use.
Should anyone want to read a narrative about the intertwined lives of Midgley, Patterson, Kehoe and lead, then this Mental Floss article is a good read. [2]
[0] https://en.wikipedia.org/wiki/Robert_A._Kehoe
[1] https://en.wikipedia.org/wiki/Clair_Cameron_Patterson#Campai...
[2] https://www.mentalfloss.com/article/94569/clair-patterson-sc...
It's a rambling article that provides no real evidence, only speculation about future discoveries (which never came) and absurd arguments why its concentration is supposed to be smaller.