I used to have the same problem...
I was young, about 5th grade and all my essay where horrible full of grammar error and I always got very bad grades on those.
Then I move in the Junior High / Middle school and I was still doing very bad error, but the content of my essays was great so I was able to get about 60-70% on my test even if, again, the content was above the average.
It was bad especially in Italian (my first language), where how you say a word is exactly the same way in which you spell it, we don't even have idea of what a spelling competition is, it just doesn't make sense.
Then I move to high school, in my first essay I didn't do any single grammar error, it was great, no error at all.
I probably do have some problem especially with pronunciation, my parents used to bring me to some kind of doctor who I remember I hated, in my mind words were (and are) fine, but they just come out wrong out of my mouth, I ramble a lot.
Some people say that my mind just work faster than my mouth so I can't keep up with words and some of them come out wrong, obviously I can simply talk right if I put a little of effort on it, but sometimes especially when I am talking just because and other person are not suppose to care of what I am saying I just ramble (yes, I talk with myself, ok ???)
People usually don't define me like a dumb person, and my ramble nowadays is a problem only when I want it to be a problem...
I know that it is very hard to say, especially at a father, but kids most of the time just need time, your son is not stupid I won't stress him with those card, if you really want him to make some exercise ask him to keep a diary it helps a LOT, and not only with spelling but also with construction of phrase and syntax that is what really shape the mind of a person.
(NB Keep a diary is WAY HARDER than play with some cards, not because the spelling or the syntax, but because is something hard to do, it takes more effort, more time and he must be very determined doing that (I am not able to as instance) but it will lead to better result, it will give your son a wonderful memories especially for when he would be a teen or an adult, and it will teach your son DETERMINATION that is the only thing can be teached in order to make a successful person)
I probably do have some problem especially with pronunciation, my parents used to bring me to some kind of doctor who I remember I hated, in my mind words were (and are) fine, but they just come out wrong out of my mouth, I ramble a lot.
Some people say that my mind just work faster than my mouth so I can't keep up with words and some of them come out wrong, obviously I can simply talk right if I put a little of effort on it, but sometimes especially when I am talking just because and other person are not suppose to care of what I am saying I just ramble (yes, I talk with myself, ok ???)
People usually don't define me like a dumb person, and my ramble nowadays is a problem only when I want it to be a problem...
I know that it is very hard to say, especially at a father, but kids most of the time just need time, your son is not stupid I won't stress him with those card, if you really want him to make some exercise ask him to keep a diary it helps a LOT, and not only with spelling but also with construction of phrase and syntax that is what really shape the mind of a person.
(NB Keep a diary is WAY HARDER than play with some cards, not because the spelling or the syntax, but because is something hard to do, it takes more effort, more time and he must be very determined doing that (I am not able to as instance) but it will lead to better result, it will give your son a wonderful memories especially for when he would be a teen or an adult, and it will teach your son DETERMINATION that is the only thing can be teached in order to make a successful person)