1. Take the glancing hit theory and throw it in the trash, the bolt hit him squarely in the lower left abdomen, possibly as low as his hip, spun him around, and knocked him to the floor
2. After this point Ren only stands straight up for moments at a time, and spends a lot more of the time hunched over and to the left, consistent for someone with a nasty gut wound as the act of standing straight up would cause the wound to stretch and tear further, this hunched stance becomes more pronounced as the fight goes on
3. He has increased difficulty in just walking as the fight goes on, by the time Rey has the lightsaber he's lurching forward more than walking, which is logical because ignoring pain, or fueling the darkside or whatever is all well and good, but pain's there for a reason, like to tell you not to move around because you're ripping the frigging wound open wider
4. During the fight he spends a large time holding and fighting with his lightsaber one handed, which hand you ask? Why the right hand, and where's his left hand when he does this? Holding his lower left abdomen, exactly where he got shot, almost like it was causing him pain or something