Once your child is an adult, his options will depend on how high functioning he is. When he has “aged out” of public school, a vocational training program may be the next best step.
Depending on his capabilities, he may excel at jobs that require enormous amounts of concentration but limited intense interaction with others, such as computer programming or graphic design. Or, he may prefer to do something more repetitive, such as filing. If he's academically rigorous, college may be the answer. (Temple Grandin, an autism activist who's on the spectrum herself, is a renowned professor of animal science.)
In short, he or she is only truly limited by his or her own abilities and interests, which holds true whether one is autistic or not.