Discuss this with the prescribing doctor.
The coughing might be a side effect of Dexilant
or Dexilant could be exasperating the coughing and causing other cold-like symptoms as well!
Since Dexilant is not helping he should get off it ,or if not safe
to do so right away, it should be tapered down.
Another issue he may have is low tissue magnesium-
forget blood test results as they are useless for practical purposes- as PPIs tend to lower magnesium levels
and if he already had a Mg deficiency from before, it
needs to be addressed.
Low magnesium symptoms are numerous and could easily explain all of his symptoms!
I will send you a link for more details on a PM.
If it were me, I would do a magnesium chloride trial,
1 oz magnesium chloride dissolved in 1 Liter of water
and sipped VERY slowly to avoid laxative effects.
Another concern is low B12 levels , since Dexilant
lowers gastric acid which affect B12 levels.
You mentioned he changed his diet.
Can you offer more details?
If he can optimize his diet, take high quality probiotics and digestive enzymes and tweak a few things such as what I mentioned above, he might be on the road to recovery.
Best wishes,
Niko