While he is in the regular classroom, such as art and music, the teacher needs to be sure that she is making eye contact with him when she talks, so that he can get the visual cues that will help him understand and process what she is saying. There should be ample opportunities for small group work, so that the level and intensity of child-to-child communication can be monitored by the teacher, who can help the kids repeat or add gestures as necessary. Imagine if your child were deaf -- he would certainly be put into regular classrooms, but the kids would have to be taught sign or other ways to communicate with him. The same is true for a child with CAPD. The other kids have to be taught to understand the condition, and the things that they can do as "good citizens" to make it possible for your child to be successfully included.
If your son's writing is affected by low muscle tone, there's nothing about a separate class that's going to make that easier or better for him. He needs proper seating (perhaps with specially designed supports), a proper writing surface (which might be a slanted slate or an electronic slate that translates his handwriting into on-screen text), and an understanding and talented teacher who believes that every child has a right to be in a "typical" classroom.
The same is true for his speech. A self-contained classroom might be a good place to get individual speech therapy, but the best way to learn to communicate better is in a class with other kids who are trying hard to listen and understand you. Your son may need a 1:1 aide in the regular class to help him navigate the demands of the curriculum. A speech and language therapist should be spending some time in his classrooms and consulting with the teachers, so they know how to integrate strategies that help with language development, when the speech and language person is not there.
The bottom line is to make the environment as normal or typical as possible, and move in the direction of specialized, segregated settings only if all of you believe that it would be impossible or unproductive to offer the services in the regular classroom. The teachers and the other kids need to think of your son as a kid in their class; not that kid from the "sped room." Your son should have a seat in the regular classroom. He should go there for homeroom, leave for home from there at the end of the day, and be in there as much as possible in between.