I would drop it. You can continue to use the EEL as a supplement (that's what I do). How is he doing with IEW writing? Do you feel you need the structure of the class to have him continue to learn and grow in grammar and writing?
I tutored Essentials and had a daughter in it. Although I still use the EEL, I also continue with R&S. We stopped using IEW, so the benefit vs time factor to me didn't add up.
I think Essentials is a service for families that desire it, but I would not consider it in any way essential for higher level classes, Challenge or otherwise. It is but one way to study grammar and writing, but you may find your time better utilized covering it yourself.
Blessings,
Lisa