When you contact the Safe Schools Advocate, be sure to refer to the doctor's excuse. Also, talk to them about alternative education options for your son. Also, ask if the school district is required to provide a home educator while your son is at home (although, he may not be ready to sit through anything resembling school). Find out what provisions are in place to move him to a different school.
Finally, help him to have fun, healthy interactions with children in his age group. This will help him to experience relationships that are fun and will build his self-esteem.