Is it not more a question of the support that can be expected by the other pupils in his class? If the child with special needs is working well but still has issues then those issues are impacting on the teachers and his classmates rather than the child itself surely?
This would be a potential issue regardless of which school they go to, whether it is a GS or comp.
Why should he be not be encouraged to go to a from GS because he has a SEN?
ADHD dad, if you feel he will benefit from a gs education, then talk to their SENCO. See if you get a feeling that they would actually be helpful and positive about your son. Tbh, that's goes for any secondary school you are thinking about. It will an important part of your school decision.