Recently I read on a message board on Facebook a question by a single mom desperately trying to figure out how to work and get all the subjects properly covered. Her son loves science, but she was feeling that science was taking up too much time and other subjects were suffering because of it.
These were some of my comments:
The love of learning is extremely important and the more you unschool - letting the child delve as deeply as possible into the topics they enjoy - really helps with that. Its not always easy and it often requires help.
I would enlist dad's help if possible in reading some science or suggesting they visit some science museums or something. Dads aren't always up for that type of thing.
Perhaps seeing if he could tag along some days at another homeschool house would also help.
We are heavy into media in my home. My son watches endless science documentaries on netflix, youtube, and other online locations.
I purchased a subscription to Educator.com. It is geared towards high school. I taught him how to use Evernote and open two windows. He watches a documentary in 1 window while taking notes in Evernote in the other. It accomplishes writing, typing, reading, science education, and a whole lot more.
I think typing is something they need to learn early.
A program like Time4Learning is something he could do on his own even when you are not there. I'm sure there are others. All you have to do is teach him how to use it and a great deal of his education could be done without your direct input. I'm sure he is not by himself when you are working, so whomever is taking care of him could be instructed to get him started and assist when necessary in using something like Time4Learning for say an hour each day or one module in 2 or 3 different subjects - however long that might be per day. There are quizzes and you can print out a report of what subjects the student studied and how well they did. So its easy to see if he is actually learning or needs to go back and review some things. We used it through the 4th grade and my son really enjoyed it. After that he lost interest.
One of my friends was in a similar situation. She is a single parent with sole custody and zero interaction with the father. She worked nights. He is very science oriented and she was able to make it work.
Best of luck!
Find out about many other curriculum choices at my homeschool website: http://www.homeschoolsantafe.com.