There is a ton of professional information out there regarding the differences between sex and gender, if he’s made a specific positive claim about why they can’t be separated then we would need to hear his argument to debunk it. If he just generally refuses to acknowledge the difference then it’s possible any source you cite will just have the authors’ “motives” questioned and be dismissed. It would be as good a use of your time to just link him to the first page of some google search results.

I hope this doesn’t come across rudely, but I have a couple bits of advice that seem relevant here: First, if this isn’t what y’all originally started arguing about it’s better to forcefully keep the other person on topic, no matter how inflammatory their gishgalloping may be—any new shitty claims they make while cornered deserve no more than passing derision. Second, try to research beforehand and feel grounded about what specifically you’re planning to challenge somebody on, rather than putting yourself out on a limb and then hoping others will help you win an argument. If you rushed into a debate out of a sense of obligation to call out bullshit, that same rationale also obliges a degree of self preparation.