“He loves spelling and can easily spell words that he struggles to read. It doesn’t entirely make sense, but I am left to think that if he is giving his cognitive, developmental attention to writing and spelling – the actual construction of words – then he doesn’t have much space left for learning to read”
As this author notes, if you can spell, it doesn’t mean you can read.
Montessori emphasized that children, who spelled with the moveable alphabet, were often unable to read the words they were spelling. Spelling in the Montessori approach is one of the building blocks that leads to the abstract activity of reading. It’s very normal to spell before you read.