Sounds like you're doing everything right as best you can. Of course, even with all the best care, sometimes genetics play more a stronger roll in their health, but the good news is, as long as it's not something terminal or severely detrimental, regular checkups can usually catch stuff early on, which is very important. That's pretty much my case with my girl though had to spend a good bit this summer since she's gotten one issue after the other (thankfully minor issues).
My girl does a sort of oink/honk as she walks sometimes and even sound like she snores when awake on occasions. This is kind of a reverse "sneeze", though to me, personally, I don't think I'd call it an actual sneeze, per say
But in your fellow's case, it could be more that his nasal passage is a tad narrow/small or like my vet once said, it could be allergies. I've had a male yorkie in the past that does it like he's having trouble breathing but snort/honks too and we found it was more of a mild asthma issue (though he's making the reverse sneeze sounds, too). It'll pass quickly if he calms down, so we usually just hold him and help him calm down. I'd ask the vet and maybe try to video it while he's doing it to show the vet or look at videos of reverse sneezes, I believe someone posted it around before, of course you can look up on YouTube.