Unexplained weight loss is always worrying. He may be holding back on some symptoms and not telling anyone. This of course is his prerogative and you are in a difficult position. Blood tests are only of limited use.
It sounds like his metabolism is in over drive get him to see his diabetes specialist nurse I am a diabetic too it could be he if he isn't on them needs to go onto insulin?
A whole box of triscuits after a big dinner?
That's over 1000 calories and 3/4 of them are from
carbs! And he's diabetic. Something is definitely going on here.
Whole wheat, white wheat, are pretty much the same when it comes down to blood glucose.
One needs to check the glycemic load, when it comes to diabetes.
Whole wheat may take slightly longer to get processed and have more fiber and natural (not added) nutrients.
White bread and whole wheat bread have the exact same Glycemic index value at 71.
--Source: Harvard Medical School--
So eating whole wheat is not necessarily doing much for
diabetes, but I guess the other changes he implemented, made the difference you mentioned.
Now the question is, since he has a metabolic dysfunction, did the continued consumption of carbs lead to candida(a yeast overgrowth infection), did he have it before and if he did it perhaps predispose him to a parasitic infection?
Which (parasitic infection) could also develop on its own and since he travels internationally, it could be a valid suspicion, as his risk to parasite exposure would be higher.
Candida and parasites are often missed by conventional tests.
The test I would ask for to check for parasites is a
Comprehensive Stool Test, which uses Polymerase Chain Reaction (PCR) and can detect parasites in their dormant state also, something conventional stool tests cannot detect.
His doctor might not be open to order such a test,
so in that case I would suggest he goes to a Naturopathic or Functional Medicine Doctor.
If this indeed is the case and your father does nothing about it, chances are that in the future he may not have a choice.
Some medium to long term complications arising from untreated parasitic infections include vital organ dysfunction as these parasites can take residence in the liver, heart or brain and cause interference in their normal functioning with potentially severe consequences.
I hope this helps but please note that my suggestions and comments are not intended as replacement for medical advice.