I'm not sure what kind of vet access they have in Vietnam, but I think the weather should certainly be comparable to Indonesia given it's proximity.
Yeah, any vet would work. Livestock, pet, whatever, the process is the same. Smear a slide with a fresh fecal and look for eggs and worm-like, maggot-like bodies under a microscope. (Very, very over simplified.) The typical medicine used is Panicur over several weeks.
If there are flagellants; gelatin, bubbly, smelly goo present. (Again, very over simplified description.) The typical medicine protocol used is Flagyl (Metronidazole).
As to weather. The Indonesian islands are influenced by ocean currents and breezes and are actually a bit cooler than most of the tropical mainland Indochina region. (Same day comparisons across the region.)
That's not to say that his posted temps are too high for Tokay. Tokay do very well in Vietnam as well. And the Indo summer nighttime low temps are about were his high temps are now. So I really do not think temps are the issue.
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Obviously we are guessing and 'playing doctor' here. I think we are all on the same page about the possible parasite load as the culprit.
I post the above information not to supplant an exam. But rather to give an expectation for a possible coarse of action.