I actually just went through a similar situation with my father.
I told him shortly after I discovered it a little over a year ago, and he was very skeptical, I kept working at him and around 5 months ago I asked him to let me post some of his work on there if he didn't want to make his own account, he said okay and I posted his photography which was received very well.

He still didn't join. Not until my account value went way up did he really start taking it more seriously. His main concern was similar, in that he thought maybe it was some sort of scam and he wanted proof and me to take money out.. Which.. I was prepared to do and was going to do.. But then I ended up just putting it back into steem, lol.

The logic I used that I think helped a lot was this..
I told him that I've never once heard of someone "not being able to cash out".. You'd think in all this time, someone would have made a YouTube video or something proving that it's a scam.. But that's not the case, there's very little out there. And I've never seen anyone who hasn't been able to cash out in that sense.

Whether it is some kind of scam remains to be seen, but.. Right now.. Plenty of people are using it to make a lot of money. And.. it might continue to grow and be something like Facebook someday, which would make early adopters be in a much better position, especially in terms of potential monetary gains into the future.

If I was you, I'd tell him stuff like that and offer to help him as much as you can. Tell him to just give it a try, what's the worst that could happen, he might actually like it. :)

Anyways.. Hope any of that helps! My father did end up signing up recently, and I didn't even have to ask this time.. He asked me. And he's been doing pretty good on here so far!
Cheers and good luck.