Hi iybuyk - I just found your post in the unanswered posts under Active Topics. I am sorry I don't have a specific answer for you, but I'm bumping up your topic so that others will see it again.
It sounds like your DS5 might be like my DS5 - doesn't like to write. With mine, I think it's a perfectionism issue - he learned early on that the letters and numbers didn't come out how he liked, so he quit trying. He writes a little bit in his preschool, but he's not interested in it one bit. Also, there is a range at when your hands get coordinated enough to actual do the writing, I think. Some recommend Handwriting Without Tears.
I don't know how much you can expect a preschooler to be self-motivated. I always hope the teachers will help with that by providing interesting and appropriate challenges.
Again, I don't have any personal experience in any of this, but as for my thoughts, i've got lots of those! I would think that being in a school where one is not the smartest would be helpful for these really bright kids, because it might help in forming study habits (if they are the smartest ones, and never have to try, they never learn anything about studying). But I think that the right teacher is the most important thing. Can you ask the current teacher for advice on how to help with the writing? Is she willing to explore different ways of doing things that might not involve so much writing? I would try to talk with her first - explain how he is obsessed with math at home, and ask if she has ideas on how to show this without the writing, which he may not be developmentally ready for yet?
Just random thoughts. People with actual experience, please chime in!