A good way to recognize dehydration is by the color of his urine. If it is a dark yellow with a strong smell, dehydration is likely. If it is a normal yellow, it has a normal amount of water in his system. I am surprised the doc did not order a urinalysis.

Another way is to pinch his skin. If it is flexible and stretchy, it has normal fluid levels. If it shows the pinch mark after releasing the pinch, dehydration is less likely. Dehydration will cause head aches and cognitive issues but they will go away soon after the body is properly hydrated.

But, to please the doc, just keep track of his intake. No need to upset the doc and get his against you.

My best to you both.