If he thinks you’re being too OCD in your observance of kashrus, refer him to the eleventh perek of Mesillas Yesharim (about five pages in, depending on the print) where he talks about ma’achalos asuros.
For a real-life example of what he’s talking about, remember the Chicago Tylenol Murders, where an estimated 31 million bottles of Tylenol were recalled in response to a total of seven reported deaths.