Welcome to the forum and to having a mini (which if you are like most of us, it will soon be plural!).
As for correcting his nipping, this is such a common problem with minis, especially the boys! I would give him a sharp slap on the shoulder or neck (never the face) and say QUIT in a firm / nasty tone. Not NO, which sounds too much like WOAH. If that doesn't help, the slapping, you could use a crop or switch. With some of my horses, a slap doesn't mean a thing, but a crop does.