We are homeschooling. I require both my kids (5 and 9) to do a journal entry and a page in whatever Writing without tears book they are working in at the moment every day. For my 5 year old, this is typically a 10 minute thing. Now my 5 year old does a lot more writing on her own. They have other minimum requirements too, but that's it for writing.
I guess my point is, if you can give at least a few minutes a day to the weakness (writing or whatever) you can usually make great progress over the course of a year without really sacrificing much. My kids have many open hours where they can do what they want or we dig into bigger projects, go on field trips, etc. So you can still make science the center of your school day. You can even have him keep a science journal where he might write a sentence or 2 a day about whatever he did.
Good luck!
