Did you mention to the dentist that he was having difficulty chewing his food when he had his teeth done? I recently had my horse's teeth done, and when I mentioned that he was dropping food, the dentist said "oh, I'm glad you told me that because that means I have to file them a different way". If you didn't mention it to him, he may not have filed the teeth for better chewing. Now my horse doesn't drop anything, but he's also a senior and it's been hard putting weight back on him. Right now I'm looking at Blue Seal Performance LS. It's especially made for hard keepers and "metabolically challenged senior horses".
I've spent most of my life riding horses. The rest I've just wasted.