Oh, right, and don't show him fashion runway shows from cons. They're easy to find examples because when you search using English terms, those are the ones that show up on top, but the moment he sees "SakuraCon" or "Japan Expo" in the title it'll only reinforce his perception. Get stuff like this:
Notice how the venue, crowd density, professional presentation, and lack of WACKY QUIRKY ANNOUNCERS give it an air of authenticity not found in what would be a costume'd runway show.
Show him documentaries that don't try to clickbait with backhandedly making fun of the fashion, but actually paint it under the light of
>you may find this weird, but we're going to dive into what makes this unique
Kawaii Pateen probably has the easiest to find videos that show lolita in a more "everyday" setting. Particularly stuff by Misako, who from the way she talks makes it very clear that this isn't just something she's dressing up for just to go to a con or parade or something. Just be sure to keep away from the actual costume stuff they have on their channel, be careful he doesn't find his way to those.
On another note, it may be dangerous depending on his personality (if he's the frugal type) but show him the storefront sites for brands. Show him how much the stuff costs, how many accessories there are, how much care is put into them, and make him question why "costumes" would be designed with such a level of customization if they weren't meant to be utilized for actual wear. Show him pictures of everyday lolitas in every day settings (check the archives, there are tons of image dump threads of these that have useful examples).
I can drop some more links your way if you're having trouble finding any, but I think I gave a good enough start for you to weave your own lolita education pamphlet for your dad. Good luck, Anon. I'm rooting for you.