Yes I have known at least one person where words "disappear"
My stepfather who is a retired minister, told me that it often happened to him. Written words would "disappear" or simply turn to gobbltygook (technical term) He didn't know until late in life that he suffered from severe dyslexia. He said that he often could read for 10-20 min. and then suddenly it was as if words would melt off the page. Thankfully for him, he also had a photographic memory which went a long way toward making up for the severe dyslexia.
I'm not necessarily saying that this is the case for your son, however, it's worth looking into.
