Hmm. Just from your list of cans/can'ts, it sounds very much like you may be looking at a 2e situation--something in the category of dysgraphia, developmental coordination disorder, or nonverbal learning disability. Are you having a neurological eval or a neuropsych done? The latter ought to be very helpful in giving you information about how best to challenge and support him.
If you search for threads on DCD or dysgraphia, you should find some good info here, as a number of families have 2e kiddos with one or both.
And I'm going to tender a suggestion based on your signature. I see that you have an adult DD who is 2e. Exceptionalities often run in families, so it is possible that there is some overlap between your DS and DD's profiles. Being careful not to expose any more of her private affairs than necessary, I will just say that you might consider asking her if she has any insight into her brother's situation.
