I really hope he can find something good to do with his time. But the others are right--it really depends on who you are, what you want to do.
What kind of transition planning has he had in school? Has he looked at possible employment options? Does he have the academic ability and desire to go to college or a certificate program? Someone mentioned apprenticeships; that's an option too. Or he could get career training designed for disabled people, and go into a job based on that.
Have you looked into the vocational rehab department in your area? They often help disabled people get jobs. One thing to remember is that "autistic" is not the same thing as "intellectual disability", and that means you can't treat them equivalently when looking for a job. Autistic people often have sensory problems that people with ID don't struggle with nearly as much, and we tend to burn out more easily on socially-oriented positions; but we tolerate repetition better and pay much more attention to detail. It's a completely different skill set, so don't let them just throw him in with non-autistics. It's got to be specialized, so he can actually do what he's good at instead of being squished into a Generic Supported Employment type of job.
Leisure--well, what are his hobbies? Your hobbies are a great bridge to friendship if you can find other people who enjoy the same things. As an autistic person and apparently an introvert, he needs a lot of alone time to recharge, but being completely isolated wouldn't let him practice interacting with others.
What's his transportation availability like? Can he drive or use the bus? If he can, he's got lots of options, especially if you live in an urban area. The library immediately occurs to me, because that's where I'd go, because I'm an absolute bookworm. But there are many options.
Building things, as in designing, or the mechanical aspects of following a blueprint, creating patterns, etc.? The first is an artistic thing; the second is more of a visual-spatial thing. There are a lot of factory jobs that involve assembling things if he's good at that. Unfortunately it's hard to find a job as an artist, but that doesn't mean he can't have fun. Legos are awesome.
Are you and he close? Do you hang out a lot? If he's socially isolated after he graduates, he doesn't necessarily have to suffer for it, if he has family who enjoy spending time with him. Even one person who treats you like an equal and genuinely enjoys your company can be a godsend when you have a hard time communicating.
Get your brother over to WP if he's interested in chatting with us. We can always use another person's viewpoint around here, and maybe he could bounce ideas off us to see if anything appeals. 