In addition to vision, I would consider the dysgraphia/DCD/NVLD category. The discrepancy between his oral spelling and his written spelling suggests that it is the insertion of the physical or visual-spatial process of writing that is the problem, and not dyslexia, stealth or otherwise. He is also very young, and may still be working on attaining automaticity in handwriting (developmentally, not necessarily as a delay), which can introduce undue cognitive load, resulting in inconsistent or incorrect written spelling, especially in connected text (versus in isolation, on a spelling test). Either way, I would keep an eye on this, and consider an OT consult or eval as a starting point.
