Daisy,
The ladies have done a great job of explaining the amount of writing expected in DITHR.

I will share that Bigger Hearts does include a mini-literature study in the Storytime box of the plans in order to cover you in the area of literature study (in case your little reader/writer is not quite ready to start DITHR). So, technically you could wait until the following year to begin DITHR, if you felt your son wasn't quite ready.
One other thing weigh, if you do go the 2 program route, is that you may wish to wait to begin DITHR until a few months in (or even mid-year), after you've hit your stride with getting the other parts of the programs underway.
Just some food for thought!
Blessings,
Carrie