This really depends on what kinds of supports your son needs. If your son has no academic needs, Landmark may not be the right place for him, since it is really geared toward students with learning differences, not autism, or social/communication differences. You need to be very clear about what kinds of supports you are looking for, and find out what they have.
Mercyhurst College, in Erie, Pa, has a program for students with Aspergers. It is definitely worth checking out. There are some other schools that also have some supports (I think Boston U, Drexel, I forget where else), but those supports are less intense and less "embedded" in the student's program. St. Joe's in Philadelphia may also have supports, since they have a lot of programs for teaching about autism - they have an autism center (McKinney ?), so you might want to check with them.
This is a constantly changing area, so information gets old fast. There are some web sites that have information, but I don't have their addresses right now. If I find any, i'll post later.