In my mind it's absolutely certain to be a terrier mix. There is zero chance that it's a shih poo. I think your guess could be spot on. If you really needed to know for some reason you'll probably need to do a dna test.
It could be a shih-poo, the coat may change as he gets older. I have family with a maltese/yorkie cross and she had wiry looking fur like this as a puppy, but her adult coat is closer to a maltese type.