I will write about my third grader, since she is closest to your son's age. She also has some "quirks" and learns best when I think outside the box. Our schedule is fairly loose. I prefer not to have a schedule, my kids prefer to have one. We have come up with something that works for all of us -- for now anyway :)
We plan on math daily. However, I will not have her do a worksheet more than 3 times/week. In reality it varies between 2-3 worksheets/week. On the other days we use manipulatives, do oral math, use the white board for problems, and on Fridays (at least) we do math games. The "Friday" games are usually for reinforcement, not new material. I will use games for new stuff sometimes, but I think of it as a manipulative then, and we do it earlier in the week.
We plan to read daily. She reads to me during the day. I vary the selections. Sometimes she reads from specific skill related readers, and sometimes she chooses. I have a couple things we use for reading instruction. That happens 2-3 times/week. Spelling is only once or twice a week right now since we are focusing on reading multi-syllable words. She is also spelling multi-syllable words (using similar strategies so it is all related). I read to her and her sisters every night (minimum).
Writing/Grammar: so far, I must be doing something good because writing isn't a chore. I rarely require much, but I allow for many opportunities for her to use her writing skills. Most of our writing requirements are about handwriting not composition. She and her sisters create their own stories and perform them frequently, so I don't worry that she isn't putting them on paper. Grammar is only done in an introductory style here about 15-20 minutes each week. I hate grammar workbooks for her, so I use one as a guide for me (topics) and create my own "grammar" activities. People read about my "egg hunts" a lot. My dd loves to hunt for Easter eggs. I frequently hide them with learning in mind. She might go find all the eggs, open them and put the strips of paper into appropriate buckets (noun or verb), or any number of things I might have her do. Active lessons are more meaningful for her.
Science/Social Studies/Art: These are usually favorite subjects in our house. Our "schedule" has Science on Mon/Wed. and Social Studies on Tues/Thurs. and Art on Friday. "Art" means learning something new related to the subject. My dd loves art and works on projects, etc nearly daily. I am not talking about that. However, if Science is particularly interesting we will abandon the 'schedule' and just do science for a while, skipping the rest. Same goes for another subject.
School takes at most 2 hours a day, broken up a bit.