The best math course I have found for a middle-schooler who is having some problems with math is the Key to... workbooks by Key Curriculum Press. They have Key to Fractions, Key to Decimals, and Key to Percents. Going through these three topics "catches up" anyone who is behind, and generally covers the concepts they need for basic arithmetic. Key to Algebra is excellent, also, for after they know arithmetic. There are also Key to Measurement and Key to Metric Measurement, which focus on measurement but provide arithmetic review. Key to Geometry is mainly drawing geometric lines and shapes, and teaches some basic geometric concepts. A compass and ruler are used alot with Key to Geometry.
For eighth grade curriculum, consider Sonlight's history studies. I haven't used an entire history package, but I used their book lists to make my own history reading list for middle school. For science, we really like Real Science for Kids. It is a curriculum that you mainly read, and do experiments about once a week. It isn't too "little kids-ish" for middle school, and gives a good foundation in science.
For one writing exercise, fact hunts are useful. Middle schoolers like to do them. You assign a topic and tell them to write down 10 facts about it. They can use encyclopedias, books, or the internet. Another good writing exercise for middle school is narrations-- they write about anything that is happening in their life, and you edit it for them, encouraging them to write more details if needed, and then they rewrite it or type it (on a different day).
For grammar, Winston Grammar is helpful, especially if a child is more kinesthetic. They learn all the parts of speech with this program, and it takes less than a year to complete it.
Some kids are easily distracted. For example, one of my sons found that our electric pencil sharpener was broken, and so he promptly took it apart and had pieces out on the table for about 15 minutes while he fixed it. He didn't get his regular schoolwork done, but at least this "distraction" was helpful! If it was me, I would just hunt for a sharp pencil, because I am very task-oriented and would want to finish whatever I was doing at present, before starting something else. One thing I have noticed is that people-oriented kids are more easily distracted by other people in the house. One of my daughters chose to study in her room, to minimize distractions. My second grader chooses to get up early and do half of his schoolwork before breakfast, because it is quieter then.
Sometimes there are nutritional issues with attention problems: you might try giving your son a high-protein, no refined-sugar diet, and adding some magnesium as a supplement.