I would agree that experimenting with speech-to-text would be a good place to start, and that he might benefit from trying a variety of graphic organizers and mindmaps until he finds one that meshes with his thinking style.
Although his writing tutor may be very experienced with GT children, the strategies that work with most GT learners may not be equally effective with 2e learners. Generally, when students with ADHD struggle with extended writing (which appears to be the case here), the challenges fall in one or more of: idea generation, initiation, organization, elaboration, self-editing. It may be valuable to observe his writing process and try to determine where the lapses are in those terms, so that scaffolding and remediation can focus on the most relevant areas. For instance, if he has a hard time getting started, idea generation and initiation are often implicated. If he can get a few stripped down points out, but struggles with supplying the supporting details, then elaboration is probably the difficulty. If there are good points, but no logical thread or argument, then organization is likely a concern. If he leaves out words or has a hard time distinguishing between critical and nonessential elements, then organization and self-editing are probably both involved.
