Among the possible core deficits in dyslexia are weaknesses in phonological processing, orthographic mapping, working memory, and retrieval efficiency. The latter two of these overlap with dyscalculia, and often with dysgraphia, as well.
I am going to speculate that a child with working memory and/or retrieval efficiency deficits might have challenges with consistently applying chess strategies, because, for example, although the patterns are in long-term storage, accessing them efficiently and on-demand, and then holding them in working memory long enough to use, occurs through an intermittent or convoluted mental pathway. Just as he needs to overlearn core skills in encoding and decoding written language to reach automaticity, he may need to practice new chess skills far beyond the point at which he grasps them conceptually, to attain automaticity, which is what what he will need to apply them consistently in competition. Practice under conditions as similar to the settings in which they are expected to be applied will also be most effective, as it will provide him with additional environmental cues for retrieval. (This is the reasoning behind studying in the classroom where you will be taking the big test.)
