Quite unusual for speech therapist to give you such a firm observation/diagnosis. Child normally will be referred to Children Development unit for assessment and take some tests.
Your son may have missed his milestones but I thot Dr in the capacity to certify a diagnosis. It is to his disadvantage if he doesn't attend EIPIC - get early intervention soon but if he is not 'officially' termed as GDD, I doubt any special school will take him in. Obviously sending a child with special needs ie GDD to normal school might not help and could hinder the child's chance to perform better or be treated.
GDD is a more serious and harsh medical term