I would second the getting a fecal done. Even though he has been dewormed, doesn't mean he was dewormed for the worms he actually has. You can take a fresh fecal sample to the vet, and they will be able to tell you what you are dealing with. It really sounds like it could still be a worm issue. I would also check with the vet about a diet. Minis are very easy to founder so you want to be careful if you are going to be introducing a grain into his diet. You will also need to watch when the weather gets nice about pasturing!