My thoughts to add something useful ended as soon as I saw it was a mini. I have no idea as I have no experience other than patting them on the head with minis.
Now, that said, I used to have an endurance horse who would drag both back toes when we walked. She did it because she was too lazy to pick up her darn feet. That mare would snap her back shoes in three weeks by wearing off the toe. I know it wasn't a soundness issue as we usually rode 10 to 20 miles a day, seven days a week and she was sound until she was 18 and had an accident (lived to be 27).
It is possible that he just doesn't feel like picking it up. I know that some horses, especially those who grew up on flat ground tend to not pick up thier feet as well as I would like.
That said, my only other thought is maybe he is not as stong on one side?