Well - this is tough. Your DS is an adult, or will be soon, and has to chart his own path (and make his own mistakes).  All you and DH can do is set boundaries around what you will and won't do (ie, we won't support you while you try all these things, and you can't live here - or whatever).

Is it possible that part of the reason why you have a tenuous relationship with SS is due to pressures DH and you exerted on him? And is it possible that you've done the same with DS, and DS is frustrated as well? Perhaps SS understands where DS is coming from, so they're connecting on that level, which would be entirely outside yours and DH's comfort zone.

I'm not saying this is the case, but it might be a good idea to look at the expectations you've put on your DS and see if they align with DS's life goals. If DS feels like he has only ever had one path laid out before him, then SS (or anyone) is going to sound very appealing, especially if they are offering a future of getting to dictate what it is that DS actually wants to do (or try).

18 is a good age to take risks. Getting a real estate license isn't a bad career move. Neither is going through coding boot camp. Both will land him a job with potentially high income. It would be better for DS to flop in these endeavours now than when he's 30 with responsibilities.

So, let DS do this without assuming it's nefarious (unless you know for certain that SS means DS harm). Screwing up at 18-20 likely isn't going to have lasting impact, especially if that screwing up is going a trade route with income potential. College will still be there if/when he needs it. And unless DS is sitting on a lot of cash that he could potentially lose, the risk doesn't seem that big.

Your SS is not wrong. There are other ways to build a life than starting with college. I would have a discussion with your son's dad (your husband?) and work out what your plan b is if your son does not attend college this fall. What are going to be the requirements for him being able to stay living in your home or will he have to move out? If you were going to financially support him in college in any way will you give him that financial support if he chooses to do other things? Is college help a one time offer, now or never, or will you be willing to give him that support in a year or two if he changes his mind and wants to go to college then?

I would try not to alienate your son too much, particularly if you susspect that your ss may be trying to take advantage of your son. My guess is that you fear he will somehow take advantage of him monitarily or get him involved in dodgy deals etc. By keeping the line of communication open and showing an interest in what he is doing you might be able to help stear your son away from making huge mistakes and trusting ss too much without looking out for himself. 

Your son is becoming an adult and is at the best time to make mistakes, he can recover much better at 18/19/20 from most mistakes than he can at an older age. Let him go and make these mistakes.  Show interest and moral support along the way and be there with advice when he asks fo it.

I am just going to throw this out there... 

I would not use a young real estate agent with no life experience whatsoever. 

He mayne can "get" his real estate licence... but can he find/earn clients? 

Going to college, even just "generals" will teach him basic business concepts - accounting, marketing, etc. as well as earning life experience. 

I think this pandemic has given many a chance to rethink how to plan your future.  Having an essential job looks more attractive in hard times.  Trade school might be a very good answer for your son. 
 

I'd get him thinking long term and what he's wants to do, see and have in his life.  Taking a job based on commissions is a hard way to live when starting out. Would college help him get to where he wants to be quicker? Would a trade school do that?  SS makes good points but they sound like short term things which could make life long success harder to achieve.  Is your son good at networking, have the necessary social skills needed for real estate/sales?   
 

I don't think you need to panic yet but sit him down and have an adult conversation about this  new lifestyle we all have and how he thinks he can come thru it launched.  

I will admit to being a university snob. My daughter, at 18, wasn't ready to be in college. Always a hard worker, but she needed another year of maturity.

But in my family we go to Prestigious University out of high school. So she felt pressured to do so, and I didn't know it or pick up on it. And she flunked out pretty thoroughly..

What finally turned her around was a general education course of study at the community college. I really had my arrogance handed to me, and I deserved it. In that program she had to take a bunch of different courses, and she found something that clicked with her.

She starts her doctoral program in the fall.

You might consider that part of his exploration include basic coursework that will transfer to universities. He'll get some of those early requirements out of the way if he does decide to pursue college later, and he'll have some time to mature a bit more.

Problem now in higher ed is that so much is online or hybrid that it's really hard to have a "normal" college experience. We're heading into a new normal, but don't know quite what that is.

Your SS is right about one thing . . . if ever there was a year to put off college and take a gap year, this is it.  I don't think you mentioned whether DS was going away to school, going to a local 4 year or going to community college.  If one of the first two, I'm assuming he would have already made his decision and talked to the school about deferring admission if he's going to take a gap year.

I would not discourage the gap year, and I wouldn't discourage him from having SS mentor him.  First, he's an adult and you're not going to be able to dictate what he does.  Second, from what you've written in the past, SS is an accomplished, educated, successful individual who might well be able to help your DS, if he's sincere about this.  If not, your DS might learn a hard lesson.

What I would point out to DS is that education is important and you fully expect him to go to school next year.  Remind him that SS went to college and earned his degree, and no matter what SS says of his reasons for doing so, he DID get his college education.  Also remind him that SS has wealthy family to fall back on, and can therefore take risks that might not be prudent for DS to take.  

I see nothing wrong with charging DS rent, nor do I see anything wrong with letting him live with you rent-free and save his earnings for his future college expenses.  I also see nothing wrong with putting a time limit on your financial assistance with college, although I would certainly give him at least a year's leeway.  Not every kid is ready to go to college immediately following high school, and it doesn't do a kid who doesn't know what he wants to do a lot of good to go when he's not ready and spin his wheels.  A year off may give him an idea of what he does (or does not) want to do with his future.  In your shoes I'd be leery of SS, but you're going to have to let your DS figure out if SS is a concerned half-brother trying to help him and mentor him, or if he has something less altruistic in mind.

It does sound like your SS may be in some sort of pyramid scheme. You could look him up on LinkedIn. If what he's doing is on the up and up, he'd have a profile and links to what he's doing. You'd be able to check. 

Is it possible your DS is getting nervous about going to college? Both OSS and DS behavior changed prior to going and I even had a conversation with my DS about fears. He said he wasn't afraid but when we hit the end of moving him in and leaving campus there was some fear in his eyes and he admitted he didn't think saying goodbye would be so hard. He was fine but he's always gotten nervous during the build up to big events.