It's pretty hard to explain this to non technical people and also can be perplexing to programmers. First, you can explain to him what currency is. Fiat currency is backed by a government's credit, and Bitcoin by a decentralized network. The advantages Bitcoin offers are all transactions are transparent, and no single authority can control its network. It would be impossible to alter the amount of coins or any past transactions without simultaneously modifying millions of computers around the world, which is also unlikely to happen. Also it has been running for seven years and no authority, even FBI could shut it down.

It is not that hard to convice people bitcoin is not used for bad stuff only. People buy drugs with dollars, at one point I read that on every single 100 dollar bill there was cocaine traces, so does that mean 100$ is used for only illegal stuff? No. So does bitcoin.

Well, it depends to you  whether you follow your father. For now, it's better to follow your to prevent quarreling with your father.
But it up to you, you should convince your father that you are not using illegal activities and do that during eating time.

It is really hard to convince someone whose at first place does not even want to hear anything new about, those who dont open their hearts and understanding for innovation abd changes. And it gets harder knowing its his father. If im in the position I will tell him that he made me a better person that I admire him fir supporting his family and I will not do anything that will put his name,our family at sake that Im doing this because its beneficial and i enjoyed it. Dont rush things for sure his dad will eventually accept it soon. Don't give up on convincing your dad knowing you did'nt do anything wrong. Soon it all be fine.

Technically speaking fiat is not backed by anything since governments stopped backing their currencies with precious metals, so the only thing backing fiat is the trust we have put behind it and if you are in this forum I suppose you don't have too much confidence in fiat either.

first show him about bitcoin is positive , say for him positive things . tell why you use it ? example because bitcoin can get more profit
i think your dad will know or may be use it too

How about this, in addition to teaching him about bitcoin - including all the good and bad things about it, why not create a wallet for him and give him some amount of bitcoins and let him try it out/play around with bitcoin for himself. Show him how to keep and use bitcoin safely and carefully and maybe he will see first hand for himself that Bitcoin can be trusted and safe if used correctly and smartly. It may encourage him to open his mind and do some more of his own research of bitcoin if he owns some of his own.

And also cash some of your bitcoins out and show it to him, it would be easier for you if you have no day job as you can show the money to him and say that this is what bitcoin gives you, you are not working but still you are earning money, and that opportunity is given to you just by bitcoin.

My father was a retired employee of the mining company. And he doesn't like to work online. No problems so far, he did not interfere in my business, including bitcoin. If necessary, talk only about virtual currency that can be used for trade, if he sees the negative news about Bitcoin in tv and internet. Notify if Bitcoin is similar to FIAT that can be used for anything, including crime. So exactly the same as most currencies.

Why do you need to convince your dad for bitcoin? Convince him for your girlfriend and keep bitcoin within yourself to get the most out of it. Earn adn hold your bitcoin for few years and buy something expensive for your dad to surprise him. Your dad will understand. You will not have to convince him that time.