Try explaining about parasites and stuff. That did the trick to convert me over. Along with my snakes i also feed my leos pinkies every once in a while (as a treat) And they have gotten round worms. Let me tell you that isn't a fun thing to deal with. So try that approch that if he feeds f/t he doesn't have to worry about parasites and the costly vet bill to go along with it. Just my thoughts. Specially if he isn't worried about his snakes getting bit by the mice. This approach might work.
1.2 Bearded dragons (Login, Raven, & Jean Grey) 1.1.1 Corn snakes(Havoc,Sable, & Kink0 1.5 Leo's (Psyloke (Lucky-male) Speedball, Domino, Rouge, Siren, Elektra) 1.0 Veiled Chams (Neo), 0.1 Rose Hair T. (Night Crawler) 0.0.3 Crested Geckos (Shiva, Storm, Beast) 0.2 Kenyan Sand Boas (Tigra & Cloak)