And this is why marriage ought to be a forever decision. You are hooked up with a guy with a wife (x) and two kids. Of course he MUST support his kids. If he does not, then he is not a man. And, why should he not make an effort to get his son to a job? Just because he is shelling out cash does not mean that he should not do anything else when it comes to raising them.
Your husband made a decision to marry a woman, lived with her, had a few kids, and then decided to split up. The kids are the ones with the grievances here. The parents made their choices. The kids had no choice. Of course she still needs the support money even if the kids spend part of the time at your home. The house payment or rent isn't determined by how much time the kids spend there.
It sounds like one of the kids is 16 at this point. The other is probably not far behind. Soon your husband can end the alimony and support. But he will still be a father, and he will still be giving his children help, providing for their education, and even dealing with his x from time to time -- weddings, funerals, etc.
The x-wife will have her situation change considerably soon enough. And your part as the girl friend will only look like a gold digger, if you try to get between your fiance and his first family while the kids are still minors/in school.
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