SM to nerdy teen boys here who also happens to be a nerd herself. Yes, people can make money streaming through Twitch, YouTube, and Patreon. Twitch allows you to subscribe to content that people make (very similar to folks who would pay for an ESPN package for their television). YouTube provides ad revenue (very similar to how traditional television makes money off ads and viewers). Patreon is more like "tipping" someone or being a more traditional "patron of the arts" who pays a set amount to support an artist so they can create content.

If your SS is a good and engaging gamer, it's quite possible that he makes money. To make good money, he'd have to have some social skills, or at least good acting skills. It's not just the playing that people watch; it's the commentary and social interaction between the streamer and their fans. It's possible that he's having a lot of social interaction with others, even if not face to face.

Keep in mind, too, that many streamers have to plot out what they'll play, create drama in what they're playing, edit their videos, come up with new ideas to keep people entertained, etc. It's definitely not traditional work, but it's the modern television for younger generations. It's replacing traditional television, but younger generations still expect it to be interesting. That has to be manufactured. It also isn't super cheap to do since you need decent mics, computer set-up, editing software, games, etc.

Should you be worried? Eh. Even if you should be, there isn't much you can do to make him change his mind. What you CAN do is make it clear to your spouse that either you or their kid will live with you after college (or whatever launch date you've set), but not both.

I'd make sure the rules are clear and consistent. He doesn't work because he doesn't need to- no additional money from DH, Move Out date, etc. 

I had a huge long response breaking down how much social media influencers (streamers etc) make because I taught a kid who has chosen to start that career before he graduated his Senior year in High School. I was just as confused about his choice because he has so much potential so I asked many questions. Basically: it is possible your stepson is making money streaming depending on how many followers he has which is also based on how good/likable he is....the way they make money is ad revenue (putting ads or doing an ad during the stream) or doing promos for products where usually the payment for this is the product for free or limited subscription which again he wont qualify for without having enough followers (Im talking tens of thousands).

He wont launch....at least not in this decade....because everyone and their brother want to be "social media influencers" and the market is now over-saturated. You and your DH need to sit down with him and let him know that while he pursues his dream career, he also needs to make ends meet and help out which includes helping in grocery money and living expenses OR show monthly savings towards an apartment or other living arrangements in the future. You both need to come up with a specific date that he will need to be out by. He can sort out a part time job working 30-35 hours at a local retailer or resturant and work around his streaming schedule. This will not work if your husband is not behind you all the way. 

I like the idea of him getting a part time job--like maybe 20 hrs a week or so? Bring this up to him in a way that still sounds supportive of his gaming aspirations. Explain how having this extra job could help him acquire the equipment and things he needs in order to achieve his gamer goals, and how it would be helpful in case he has a slow week for gaming income and needs to pay bills. To you, this gets him out and doing something in the world. To him, it sounds like you've put some thought into it and actually understand and want to help. It wouldn't hurt to do a little research on some notable gamers and find out how they started. A lot of big name influencers had regular jobs for a long time before they were able to start making a livable income from gaming, youtube, etc. And a lot of them share their stories and/or helpful getting started tips too. Just being a little more educated on the subject will give you a better leg to stand on with him, I think.

He 'says' he makes good money streaming.  No more telling, time for some showing.  Demand that DH sit down with him and review all of his bank and financial records to bare the BS he is invariably slinging.   If he isn't making any money, shut down the internet and introduce him to the local landscaping contractor.

It is cool that now you can make money on video games. I am playing now with my friends in valorant, it is an amazing first person shooter. It looks like CS GO but is much better. We play online and we are thinking to open a channel on youtube about that, we bet on money or play on wishes and it is very funny, I think a lot of people will watch that. I am sure a lot of you are playing in valorant so you can use https://valorboosting.com for improving your game account.

He will be living in your basement playing video games until he is 30. Doesn't matter if he does well in school, but doesn't apply himself. 

When SD was younger she was beyond obnoxious about playing roblox on the ipad. DH didn't enforce any screentime. I turned the wifi off to the device. She of course didn't get it. Would stomp off when it "quit working" and finally went outside. Probably not the right thing for me to do, but hey, it got her to exercise. DH certainly wasn't going to make her. Your SS is probably too smart for that though.