I'm not a min pin owner or breeder, so I may be corrected, but I'd guess he's either a mixed breed or possibly an out of standard min pin. A breed standard is basically "this is the perfect example of this breed", what you see in the show rings, so when I say out of standard I just means he has features that aren't typical of the breed. He has a broader face, white on his chest, floppy ears, and it looks like a tiny underbite? None of this means he's a bad dog! These features could be because he's a mixed breed (which could include min pin in there), or that whoever bred him wasn't taking care to breed dogs that were as close to standard as possible.
I'm leaning towards a mix of some kind, but honestly there's no way to be absolutely sure. A DNA test from a reliable company (Embark is my favorite, but Wisdom Panel is also decent) may get you a better idea of what's going on with his genetics, but even those aren't always 100% accurate and can't tell you about his ancestry beyond his great-grandparents (so even if he comes back pure, it's possible he had a great-great grandparent that was a different breed).