Spelling incorrectly can actually be a little dangerous for certain kids - I'm thinking particularly those with unusually strong visual memory. While for most kids, it's a process of learning to gradually morph their spelling into correct forms, for others, a word makes a visual imprint on their brain and its extremely hard to unlearn an incorrect spelling.
So it's not entirely impossible your DS may resist phonetic spellings because he recognizes they cause him a problem. Such is certainly the claim of my hyper visual spatial (and dyslexic) family, for what that's worth. They describe their way of learning words to me as storing pictures of whole words (not collections of letters/ sounds/ syllables). They can't change or fix parts of it - because it isn't made up of parts. And once that picture is stored, it's extremely hard to dislodge.
On a practical note, if an old-fashioned dictionary is too slow for everyday writing, could your DS be allowed to use, or consult, an iPod/Pad with word prediction, that would allow him to check spellings as he goes more quickly?
