Hi,
Have you had him sexed? There are other more fun reasons for a ball python of that size to lie inverted but they rely on it being female.
Also they can sometimes use soaking or lying in strange positions to help relieve pressure from a large meal.
You could try feeding him a small F/T rat every seven days instead - the amount of food would be similar but he would never have as "Full" a feeling so may not soak.
Just a couple of ideas.
dr del