Teachermom- I have been praying for you and thinking of you since my last post. I still feel in my heart that Bigger Hearts is the right way to go for your ds - but that is a decision best made by you because you know best for your ds. You can completely disregard my opinions here, but I will share the thoughts I've had. I know how it feels to be thinking your dc may be behind. All of my dc were quite premature - 2 very premature. I constantly felt that they were behind in many things, and they were according to doctors. I became very preoccupied about each of these things - spending lots of time, trying to work on those skills with them - many tears and frustrations. God taught me that they will grow in skills in His time, not mine. I can do what I can to help, but I can't fast forward the process. He also taught me to take joy when the skill does come - because it does over time.
It might be good to explore why you think he's "behind". We all have different concepts of what that may look like. According to the age ranges for HOD guides, he wouldn't be. I agree he needs to improve the amount of writing that he is doing, but I don't think he sounds incredibly behind in that area. I just want to encourage you with that - I taught 3rd and 4th grade previously, and there was always a third or more of the class (mostly boys) that fit the description you're giving. It seemed by the end of 4th. grade - beginning of 5th. they seemed to come into their own more with writing, and be able to handle more of it. You have an obvious talent for English/writing, which makes it hard sometimes to understand why it's not going that way for ds. I have had it with grammar and math - 2 of my favorite subjects. It is hard for me to understand when they just don't seem to get it, and I get frustrated more easily in those areas with them.
If it is the history cycle not being finished, that is easily fixed, either by skipping it, by doing 9th. grade with the end cycle, by doing an extra year of homeschooling (something we're considering anyway), or by assigning free reading books and audio books that cover the last time period. He will get it again in high school anyway, so that time period will be covered one way or another.
I think if you can look at this as a working on skills year - a year to get him ready to be more independent in skills - then it will go well. Just putting him in a higher level program but then working to make it much easier won't really meet his needs. You are wise to identify this as a hand-holding year, I felt that way with Bigger too - and then the next year will be a year for him to fly! He will have strengthened the skills you want him to, and he'll be ready for what PHFHG asks him to do. He'll feel good about it because he'll be able to do what it asks him to do independently on his own.
I have a niece that was always put a year ahead of where she should have been placed homeschooling, and she STILL feels she is behind. It is sad that she thinks that because the work was always hard for her - she's actually not behind. Children think differently than we do - they aren't thinking of history cycles, etc. - they are thinking of the day to day; and if every day seems hard, and every day seems to have to be made easier for them, that's when they think they are behind.
Please do not feel guilty about not doing Sonlight. Both of my sisters had to stop doing Sonlight too - for exactly the reasons you are describing (and a few more). Equal weight needs to be given to providing free time, time to explore talents and interests, and time just to be a kid. By saying no to Sonlight, you are saying yes to those extremely important things as well. You can continue the book basket idea with HOD. We have an "Independent Reading Shelf" that functions just like a book basket. I stock it with what I want them to read (considering their interests as well), and they can choose to read from it whatever they want. This has been fun. I've used HOD's Bookshop books and Sonlight's book ideas too; I've also put audio books on the shelf, as well as any books I've found that fit our dc's personal preferences. So saying yes to HOD won't mean saying no to the book basket.
I think you'd find HOD requires less time. Simply having everything you need in your home - the books, the science materials, the history activity resources, the hands-on items - will cut down on time. There is no need to plan or prep for HOD. Just read the introduction of the guide and have those things in place before you begin. There is little flipping around in the guide. Here's the big timesaver in my mind - each box is balanced and designed to take the SAME AMOUNT OF TIME EACH DAY!!! Carrie has thought of that when planning. Even the history projects are planned out this way. You will never come across something like: "Today use boxes of all different sizes to build a realistic castle, complete with a moat and drawbridge. Paint your castle and re-enact the story you read today." (as I came across this in another curriculum's plans I used, that I won't mention the name of - and the last 2 weeks, that part of those plans had taken 15 minutes - so that's what I'd planned for it). HOD has balance. Each day takes about the same time. Each day has about the same balance of workload.
I know you'd love HOD. I think you should give it a try. You deserve to have a year of having your cup filled rather than depleted. Your ds deserves that too. I want you to LOVE homeschooling, and I know HOD can do that for you and your ds. Let me say that you will not hurt my feelings whatsoever if you choose to throw all of this out the window and go with PHFHG - I've prayed about it, and I know you will know what to do. God will reveal that to you one way or another - keep praying, and I will too. And if more thoughts come - don't hesitate to share. We all want the best for each other here.
In Christ,
Julie
With that being said, I can't advise you for sure on whether Bigger or Preparing would be a better fit, without a bit more information. 
This would allow you steady progress forward in all areas.