That was a super long one. To me the easiest way to get my mom and dad to buy Bitcoin is by simply asking them to send me money for something and then use the money to buy Bitcoin and yet tell them thats what I did. It will save you many stress of explanation. Most of them wouldnt have interest because they are stuck know it used to be.
Typo Edit
Hmm, I didn't agree with your opinion because because Bitcoin is a volatile asset and by investing money in it, you can make profit as well as loss. Thus, if you take money from your parents and invest in Bitcoin, what will you do to your parents if you lose? On the other hand, your parents will also see Bitcoin as a ponzi scheme, although the reality is not like that.
So I think permission should be taken from parents. Because he has money, if they give permission, then fine, otherwise, I don't agree with this stance, and if possible, convince him to say that Bitcoin is a good investment asset, by investing in it, high margin profit can be organized blah blah. In addition to these opinions of mine that I have stated, everyone else has their own opinion on how they treat parents.