I'm not sure I'd add anything to a 12-hour day - especially working on skills that don't come easily. It depends on the child of course, but that would have been more than my dysgraphic ds or my dyslexic dd could take at 7, plus I'm guessing that at the end of the 12 hour day you might be tired too
I'd probably try to focus on doing a small bit of work on the weekends. Make it "fun" as much as possible - I like DeeDee's idea of a journal. If the writing on weeknights was too much, maybe he could tell you what he did, you scribe and he writes or keyboards it out on the weekend and adds a picture. Or he draws the picture at night and adds the caption on the weekend.
Reading is the one thing I might try to do each night - but again, it depends on whether or not he's resistant to it. It's also the thing I'd worry about keeping up over the summer more than the writing - but that's based on future skills needed and skills most likely to be lost anyway, if that makes sense. Most dysgraphic kids are going to have to depend on keyboarding or some other AT in place of handwriting, so I wouldn't worry so much about regressing in actual handwriting skills. If your ds needs help with components of written expression, you can practice those on the weekend or with a combo of "light" night scribing and a bit of weekend work to reinforce the skills. The thing I'd worry most about in terms of regressing is reading - I can't tell you how huge the impact of dyslexia has been in my dd's life - your ds may be totally different, but constant dedicated work on reading skills was really important for her, and in spite of the many gains she's made over the years, every single bit of delay she had with reading skills impacted her vocabulary relative to her peers (because she wasn't reading for pleasure like her peers were and still isn't reading for pleasure at the same level/rate), and that's really showing up now that she's in upper elementary. During the week reading *to* him might be the thing to do, a story before bed or when you get home etc.
Will he read out loud to you? If he will, maybe early morning have him read a short story or something out loud?
Whatever happens with the rest, the camp sounds great!!! It sounds like he is going to have a wonderful summer
polarbear