My son has all of those and a few more, and he has a fine motor coordination disorder (still can't ride a bike at 13), dyslexia and dysgraphia.
Focusing on his written output and trying to improve it through conventional methods is likely going to hurt more than help. It wasn't until he was finally diagnosed and started therapy (Wilson reading program, speech therapy, occupational therapy) that things improved. The other major factor for improvement was age - at the beginning of 7th grade we saw a huge jump in his ability with written output. That being said, no amount of studying or practice has helped with spelling, memorizing times tables or with lessening his calculation errors in math.
The losing weight and nausea are classic signs of anxiety, too.
If it were me, I'd get him into a specialist such as a pediatric neuropsychologist to get a diagnosis and start the process for getting an IEP with accommodations in place. But be sure the specialist has some expertise in this area aa they're not all cut from the same cloth.
