Basically, you have no way of knowing if his mini size is due to genetics or to some other factor. It could be due to poor care early on (I'm assuming you didn't raise him from an egg). It could be some deficiency in any aspect of digestion or growth hormones. It could be a genetic mutation that happened at any time from the parent's reproductive cell formation through to the embryonic development. It COULD be anything, really.
Also, he could have other, invisible genetic defects that go along with his size. (For example, blue eyes in white cats are linked to deafness. When there is only one blue eye, the cat is often deaf on that side as well. Also in cats, the calico coloration is fatal in males). I would not suggest you intentionally breed an animal with unknown genetics, particularly because he does have an abnormality. As you mentioned dogs, I will point out that the selective (in)breeding of canines has resulted in almost every breed having it's own health issues. When you mess with genetics based on looks (phenotype) or behavior, you have no idea what effects you're having on the genotype, and therefore what messed up animal you may end up with.
