Have you looked into ulcers? We have a lesson horse at my barn that we treat for ulcers and the way we know they are bothering him is he starts refusing to do work. He will be going along great and then for no apparent reason he will slam on the breaks and refuse to move forward. You have to turn him like you are going to ask him to circle to get him to walk forward again. He will do that a couple times a lesson and then be fine for the rest of your ride. The training that you sent him away for could have been really stressful for your horse and upset his GI track. I also treated my 18yr old TB for ulcers this past summer. He is usually a horse with a huge personality and very curious about what is going on ALL THE TIME. I noticed he was extremely sluggish when I rode and on the cross ties he seemed very depressed almost. He was not as curious and his personality went away. The vet suggested ulcers and so I treated him with gastrogaurd and gave him time off while he was getting the medication. Once he was done getting the medication he was back to his usual curious self with tons of personality and was practically running away with me when I tried to ride him again.
This won’t help dragging his toes, that very well could be a soreness issue, but as far as his personality being different, it could be his gut.