Exactly how tall is he? How old is he? This would make a difference, as to whether or not and in certain registries, if he could be classified as a Miniature. Can you post pictures maybe? He might be a cross Mini/Shetland. This cross is not uncommon. And thank you, for saving a horse from Camelot.
Lets make one thing clear here. Minis are NOT dwarfs. They are also NOT ponies. They are Miniature Horses
Regarding dwarfs: Some Miniatures, Mini/Shetland crosses, can and do, produce dwarfs. There is also dwarfism known in a few large breed horses. According to the type of dwarfism - and there are several, their quality of life can be fairly easy or incredibly difficult, to deal with. Some will take a great amount of veterinary care and others, not so much. The worst cases will often die or have to be put to sleep.
Some types of dwarfism, is difficult to spot. Some, very apparent at birth. You can see some here... Our "Special" Friends
You can also read a great deal about dwarfs and dwarfism, on the Li'l Beginnings Miniature Horse forum. Lil Beginnings Miniature Horse Forums