I have some experience with this, but I'm not in the US, so I'm not sure about what kind of support is available where you live.
Is he graduating from high school or from college? If it's high school, is he planning to go on to college? Is there any kind of support available on campus for him? If fulltime college is too much, are there any parttime / distance learning options?
Are there any nonprofits already that employ people on the spectrum? Where I live there are dozens of them, there might be some in your area too. Does he have a particular strong interest that he might be able to develop into a business or career, with necessary support or in partnership with someone else? I know someone who turned a childhood preoccupation with a certain type of machine into a repair business. The business is co-owned by a sibling who handled the business side of things and they are doing well.
What is important is to make sure there's a longterm plan for housing, income and care. A close relative of mine is on the spectrum and they are in an assisted living facility. They are doing very well there but as it's funded by the local authorities, the idea that they might withdraw funding at any given time is a huge source of stress. Their parents are now getting their finances in order so they can buy their child a small apartment if they get kicked out of the current facility. Where we live it's impossible to find cheap housing on the free market and getting a place in a social housing project can take 10+ years.
As for income, they are on disability benefits and currently looking for work. A wealthy person I used to know bought an annuity for their child on the spectrum so they would always have an income and didn't have to rely on the benefits system.
Care is difficult to plan longterm because the government tends to change the care system every few years.
I think it's very difficult to plan ahead for this kind of situation, because it's impossible to know what kind of support will be available at any point in the future and it can also be very difficult to assess what a person will be capable of in the future. I know people who seriously struggled in their teens and that no one expected to ever be able to support themselves, but 15 years later live a life that's very similar to that of their neurotypical peers. I also know people on the spectrum that did well in high school and struggled later in life.