I agree with Tansy, and wouldn't buy the extension package for a struggling reader. But the deluxe self-study package looks to be real close to his reading level, so I think it'd depend on whether or not you think he'd need some extra independent work.
I definitely would go with Preparing over MFW's CtG. Your son would be at the lower end of the scale for CtG, which could make it tougher. But he'd be at the upper end for Preparing, which has the advantage of giving him a feeling of success, especially when he has enough trouble with his reading. Carrie has explained to me that she finds it easier to teach to the youngest and add-in for the oldest, rather than teach to the oldest and have to condense everything for the youngest.
Preparing is an overview of world history in one year, if I understand it correctly, which I think is great for this age, and the books Carrie has chosen include Biblical history so you get a well balanced overview, from what I can tell. It looks to have a very Biblical focus throughout. I looked at MFW for 3 years, and planned to go with it for Adv, but then I found HOD and it has much more Biblical integration. Plus, I always liked the books MFW chose, but HOD's are even better. I have the Aesop's book that MFW has, but the one that comes with Preparing is so much more appealing that I ordered it at the bookfair, because I just liked it that much more.
Just my 2 cents. HTH
Teresa