The fact that he is completing his assignments and turning them in each week shows that some work is being done. Did you know that when a public-schooled child is ill and requires a tutor at home, most school districts state that one and a half hours of home study is equivalent to one week of public schooling? So theoretically, if your stepson is motivated, he can easily complete all of his work in much less time than traditional school, and still have plenty of time to sleep late and have a life.
If you'd like to learn more about the different ways kids homeschool, I suggest Linda Dobson's book, The Homeschooling Book of Answers.