Hi bk1,
I think handwriting trouble can really effect how a kid feels about writing overall. Can you ask for keyboarding, or at least half the time? Open-ended assignments might be overwhelming (aren't they still concrete thinking developmentally...write about a specific topic, fine; write about anything--blank page). Perseverence, practice, patience, improved manual dexterity, maturity will probably improve this.
I agree with acs to play up what your son likes and does well with. I wouldn't say completely focus on these, because math is an issue, especially if he's underachieving and feeling he's no good at it. Maybe he needs to be advanced in that (one teacher told me that when faced with the need, kids learn their times tables practically overnight). If the school won't do it, maybe do it yourself, with fun stuff at first like math games, computer math games, puzzle books. My strategies (which you'll have to ask my kids if they were at all effective) were to keep reiterating how cool math is and how able they are. I'm sure a lot of others on this board are better able to recommend resources, but I liked a book on Mental Math (especially if your son doesn't want to write) and number enigmas. I think schools make it too dry & dull and there should be more hands-on (maybe a tutor could help with this)--my DS who doesn't love math really loves working with those math cubes and "Alge-blocks" I'm sorry I don't know their proper names, but maybe someone else does? If things progress positively, work in curriculum and maybe work toward a goal (try a math competition in 4th grade, like Math Kangaroo).
These are just ideas from a novice...maybe pink panther and others will see this and respond with a professional advice.
