abrightmom,
It can be a challenge to find the right fit for our kiddos, and there are many skills and considerations to weigh in correct placement. A few things in your post stood out to me that I typically would say are indicators in helping find correct placement.
Does Bigger zero in on oral narration skills? My son CAN do the physical writing but I am beginning to wonder IF extensive practice in oral narration would lay a better foundation for the written narration.
It is true that Bigger Hearts does a great job of laying a strong foundation in oral narration skills, and CM would say that strong oral narration skills are a needed prerequisite to writing written narrations. I would agree. Even kiddos who come to a CM education much later in life need to first develop good oral narration skills prior to being able to truly write a great CM-style written narration (which differs quite a bit from simply writing a summary). Children who haven't had much of a foundation in oral narration often downshift to writing a summary instead of a true written narration (in which the child borrows words and phrasing from the author to retell the reading in the style of the author).
There are also many other foundational skills that are developed in Bigger Hearts that are so helpful in Preparing Hearts and the guides to come. If a child is on the borderline between the two programs, I will often encourage the parent to do Bigger Hearts. This is simply due to the huge amount of foundational skills contained within that guide. It is an important teaching year in the HOD line-up and can only be skipped if the child is truly ready to take on all of the skills listed on the first page of the placement chart for Preparing.
He is also a slow worker. He is smart and very capable. He has a good attitude most of the time (praise the Lord). But everything just takes him a long time. I am trying to imagine moving up to full speed.
Another indicator for correct placement is work speed and ease of completion of work. If a child is a slow worker by nature, then it is almost always better to place that child in a more manageable guide workload-wise for the long haul. Our guides are very meaty, so meaty in fact that we have many highschoolers doing our guides from CTC on up. With this in mind, your child will still be challenged content-wise with the material in Bigger but will also have a bit lighter writing load to allow for work in other needed skill areas as well. Placing a slower working child in a more challenging guide just because he can handle the content isn't always a good choice, because if the activities take all day the love of learning and enthusiasm for the work will be lost (and you will find yourself either pushing the child to complete the daily tasks or downsizing the tasks... thus losing the rigor of the program).
I would lean toward placing your son in Bigger Hearts. If he is a slower worker, I would weigh whether the extensions are needed. I think as you make your way through Bigger, you will be amazed at the progress and growth your child will show. By the time, you head back into Preparing Hearts, your child will be well prepared for what is being asked of him and full-speed won't look daunting anymore!

Full-speed Bigger Hearts is actually more rigorous than half-speed Preparing Hearts.
My own very academically strong son is at the middle age range of all of our guides. He completely thrives each year, and yet has time to do the activities up to the hilt. His placement hasn't held him back in any way, and he loves his school each and every year. While he could definitely fit skill-wise in the youngest age range for every guide, I love having him in the middle range instead. He seems to make so many connections and take so much to heart!

I know children are all different, and you will know your child far better than me. I just wanted to share a couple of things that stood out to me in what you've shared so far.
Blessings,
Carrie