Ugh! Maybe you should show him this site one of these days -__-.. I live in a tropical country and I still need heaters for my bettas, they're even more important than a filter according to me.. Why don't you ask for a Kritter Keeper, they're plastic, cheap, light and you can get it in so many sizes! Any betta will be happy :)
Yeah, I'll also convince my dad if I can. The issue is, my dad used to have a tropical fish tank, and now he thinks he knows everything about bettas. He thinks they like 69 degree water. That's why I do the water changes.