What a cutie! Do you know what mom was, or if the breeder works with any other breeds/mixes? It seems pretty unlikely he's a maltese/shih tzu mix with that hair and face for sure, and I'm wondering if his litter might've had more than one sire.
I can see Shih Tzu in his overall conformation, but he sure doesn't have the amount of coat I would expect from a Maltese x Shih Tzu cross. It also doesn't look as crisp and wiry as a terrier coat.. His face doesn't look like what I'd expect from that cross, either.