Review of rescue horse after dentist visit. He's not dropping food and holding onto his weight

He's losing the same amount of food as a healthy horse.
I've added a "stress" dose or loading dose of pre and pro biotics to Arthur's feed. That should help his gut. I'll give him a normal dose for a few days.
Arthur's been shivering in the rain, be mindful it's the middle of summer here, so I'm reluctant to rug him, as it can break out hot very quickly. Penny (his herd mater and our old girl) also shivers in the rain. They have plenty of places to get out of the rain and graze, but I feed them in an area that isn't sheltered. Usually shivering is not a problem, as horses will have enough weight to cope with the weight lost through shivering, with such an underweight horses, if it was to drop in temperature overnight, I will rug him.

The video of him getting his teeth done https://youtu.be/NcwvhWPKyGo