I too have found it is easier not to combine, and we are enjoying our homeschooling with HOD so much!

I'm assuming you checked out the placement chart for dd and have peace about placing her in Beyond.

So, focusing on your ds, the writing is an important consideration. Writing skills are an integral part of learning as dc grow and mature, and writing is one of those skills that honestly has to be gradually gained over time so dc truly can become proficient at it. It just can't be rushed, as much as we moms so wish it could sometimes! Having 3 sons myself, I do believe that these fine motor skills do just often come later to boys. As hard as it is to do, I would try to throw out the historical time period in this decision, and instead focus on whichever guide best fits your ds skillwise. Choosing based on proficiency in skills will make a good fit for not only this year, but also for every year to follow. It is what will help ds gain independence, and also what will help free up time for you to teach new skills each year as he matures. If dc cannot do what the guide is asking them to do well this year, they will not be able to do what the next guide is asking them to do the next year, and around and around we go.

Here are some threads that describe the writing in BHFHG and PHFHG...
Amount of writing in BHFHG:
viewtopic.php?f=6&t=3005
viewtopic.php?f=6&t=1516&p=11829
Amount of writing in Bigger vs. PHFHG:
viewtopic.php?f=6&t=3602
Another good thing to do when trying to choose between Bigger Hearts and PHFHG is to print off the first week of plans for each of them...
http://www.heartofdakota.com/pdf/BHFHG-first-week.pdf
http://www.heartofdakota.com/pdf/PHFHG-sample-week.pdf
Read through the plans and see if your ds can really do all that is asked of him to do in each box of plans. When reading through the PHFHG's plans, especially carefully read through the "I" boxes, and see if he can read the material, follow the directions, and complete the skills independently in a fairly timely fashion independently. Likewise, consider the "S" boxes, and see if he can be semi-independent with those, with you helping him get started, but with him finishing quite independently with success. I would be more prone to place a child lower than higher in HOD, as the academic skills are incremental and and an important part of the learning progression from year to year. Can you share your thoughts when you get a chance about all of this, and then we can all chat about options better with you? Of course, prayer is the very best help in finding answers. I have prayed you will have clarity in your decision of what is best for your dc - HOD provides a wonderful way to homeschool our dc - but placement is our best friend in getting the year off to the right start! You are wise to spend some time choosing carefully. Hope something here helps!
In Christ,
Julie