He sounds a bit like my son (now 40+ ) was at primary school. He went to Oxbridge and is a uni lecturer.
What helped him most was me constantly explaining to him that many English words have stupid spellings which just have to be learned, even though they make no logical sense - that he was getting things wrong, not because he was stupid, but because the spellings were.
Not all words have stupid spellings, but look at these inconsistencies:
live - drive
back - mac, anorak
most - toast
only - lonely
work - were.
To anyone with a logical mind such irregularities are simply mind-blowing. My son's best friend who is now a doctor used to be made really angry by English spelling too.
It might not work in your case, but u could try, if he will let u, to draw his attention to the tricky bits of words like miGHt (bite), wOrk (were), yoUr (or, more), coULd (wood, put, woman) which tend to cause spelling problems. It helped my son.
To many other dyslexics my explanations have come as a huge relief - i.e. somebody understanding what their problem is.
For adults who read words as wholes and mostly just let their hands do the writing, it's not always easy to see how insane English spelling often is. But nearly all misspellings are due to its inconsistencies, although the irregular spellings can make children unsure about the regular ones too: move - groove, rude - mood, head - bed .... They are like spanners which keep wrecking the main system.
Masha Bell