PhDs aren't immune to bias. Whether or not they are willing to acknowledge and disclose them--different story. Peer reviewed journals, best practices issued by NIH NIAID and CDC are excellent in citing credible sources that would defend nicely against bias. If you haven't done so yet you may want to consider reading the exact breakdown of how anaphylaxis occurs written in the blog of a board certified pediatric allergist who is also a parent to multiple children with multiple LTFA. Know that well and then you may inquire firmly yet civilly if he is aware of the mechnics if IgE mediated allergic reactions and anaphylaxis. Present sources as being inside this specific medical discipline under autoimmune.
To ensure a proactive dialogue is in place make it about the data, the material, not between the two of you. Treat it as though you're sharing a source and allow a credible source to challenge it in like. His PhD training might force him to challenge his own bias. Howver this is a personal matter so sometimes ego and emotion can present formidible barriers.
Depending on how detached you can remain some possible time buyers might be cite your source, how did you arrive at that conclusion, please share that source with me I'd like time to review it together in detail, can I ask what that is evidenced by, let's discuss controls first so we know our data would be less contaminated, ad infinitum.
This happens with MDs, too. I have friends who go way off map for their own kids allergies because they're derms, pediatric ER, research specialists. Believe it or not it's much easier to deal with a skeptic whose patterns have been established through academic rigor, you just follow that path. It's the Neo-luddite junk science junkies with circular logic that are nigh unreachable.