I agree with letting your son join Steemit.
It is definitely an opportunity and I wish I took some of the opportunities I had when I was 13.
13 years old, with social media these days if he doesn't join Steemit he's bound to run into internet trolls one day anyway.
Steemit is not as bad as some other communities.
I definitely think it's a better idea to spread a portion of his investments, such as to Steemit, than letting all of his savings stay in a bank account.
If he wants to join Steemit as a content creator, let him experiment on what kind of content he wants to tailor and maybe help him think of ideas. We have successful users from artists to photographers to chefs and there are SO many more niches yet to be filled.
Being successful here follows the same rules as how to be a YouTuber for example.
It's a lot of work if you want to be successful.
Your son should know this also takes effort, but he could potentially be set for life like you said.

Edit: But I would be careful here, you wouldn't want your son to assume he can make a living off Steemit, if he is serious one day about being a content creator in Steemit, you should work out ahead of time an agreement, I would say something along the lines of, if he's not making at least minimum wage on Steemit by the time he graduates high school, he better not say he doesn't want to go to college and wants to work on Steemit full time. That is a dangerous line to tread and he should definitely continue education, like you said you regret not continuing education, he can do steemit part-time (which may in-turn become full-time in the future after he has stable income from his main career) while pursuing whatever he likes in college (for stability).

You find your niche for your "channel", you employ tactics to get views and gather followers.
But like reddit or 9gag, it is easy to get lost in Steemit viewing blogs all day, careful not to get too lost in it.
Best of luck :) I'll be following you and your son should he make an account one day!