Your dad's not trying to be a dick, he's simply worried about you. It takes one simple mistake of inexperience or even just poor luck to break your neck. Putting your body upside down exponentially increases this risk. Talk to him about it, voice your opinions about practicing. In the end, there's not much he can do to stop you, but if something DOES go wrong and you did it without him saying ok, you'll feel like shit.
Also, in that scenario you'd be fucked up too.
Maybe ask your dad to come to a session with you at tramp city or whatever your town has? Show him you're capable.
They call me Captain Obvious.
"Fuck them in their asses with dildos made of lava bees."