My son has a similar discrepancy between his verbal capabilities and his processing speed. He has some other "E"s, as well. For our family, we have found it easier to homeschool so that we can provide the right level of support for the disabilities while allowing him to go as far as he wanted as fast as he wanted to support the giftedness. I don't know if that is even an option for your family, but it has worked well for us. Having a scribe, voice-recognition software, and/or keyboarding for all written work, as well as extended time, so that he can get what is in his head down on paper, has been incredibly helpful for my son, who was convinced he was stupid because he couldn't write like other children. If your son is dyslexic, the options that don't require him to know how to spell words, such as a scribe or a voice recognition program like Dragon Naturally Speaking, might really be a godsend for him, particularly if he can get permission to use them for school. I second the recommendation for Orton-Gillingham-based reading instruction methods for dyslexia.
Therapy, therapy, therapy to work on skills while he's still young, but also giving him a chance to really learn things at the level he is capable of is the strategy I would recommend. Think about using audio books, videos, programs like
Learning Ally, if you have something similar in France, and just plain reading aloud to him: not only his textbooks, but books at his conceptual reasoning level (fiction and non-fiction), and giving him a voice recorder so that he can get his ideas recorded even if there is no one available to act as a scribe at that moment.
It's late here, and I think I may be babbling, so I'll stop. I hope some of this was helpful.
