I like this compromise between traditional and home school options, generally. I think it could work well as you will be his teacher. . .IF you take the opportunities that are available with local home school groups for music or art instructioon or PE, Science, social studies as well as field trips and other options to expand his educational experiences. YOU can have a lot of say in his actual educational growth. YOU can set lessons that teach him critical thinking skills. Help him find the group of kids that he fits with (just one or two good friends is all anyone needs) OUTSIDE of the org. How involved will your wife be? Who is supervising him day to day? If it is primarily your wife, this idea is not good.
I am always happy to see parents who are so involved with their children's (and stepchildren's) education. If your wife is an uberdub, beware that he will likely be dragged into FT pioneer work as soon as allowed and will likely be bullied into Aux Pioneering the minute he makes the choice for HS option.
I used to teach English to college level students and my literacy when posting on forums does NOT always reflect my abilities. Don't worry about your ability to help your son academically. They have instructional materials that I am sure you will be able to keep up with:)
It is the keeping up with it that will be the challenge. Be consistent, don't let him slack or end up playing on the computer or watching tv or reading graphic novels or MBoBS-there will be a great deal more time in his day even after he has finished with his academic requirements. Schools waste a lot of time. Do your best to enrich his education with that extra time. It is a great opportunity so long as he doesn't get coerced into FS all the time.