We use this On-line Science Textbook and like it a lot.
We usually do two 90 minute classes a week. I read the lessons before-hand and prepare questions for him in our Science notebook for reinforcement. We recently finished the first unit on Life Science. I wrote up a 50 question "test" for him. He studied from his notebook, very seriously, and got 49 out of 50 right! When Dad printed the test out, he realized that he didn't know half the answers himself. One of the best things about homeschooling is that the whole family gets to learn!