In general, your son will probably need some accommodations in the classroom. He may need to be encouraged very early to use a word-processing software program to write stories. He also may need assistance with projects or activities that require fine motor skills. Talk with staff members at the school to determine what accommodations he will need and ask that they be included in his IEP (Individual Education Plan).
It may turn out that your son's biggest problem will be his own sense of frustration. You indicate that his level of processing is far below his cognitive ability, which means that your son will be very aware of the things that are difficult for him. By identifying this problem early in his school career, however, you will be able to get things in place to help him.