Yes, diabetes mellitus (sugar diabetes) is my initial concern with these symptoms.

A simple finger prick for a blood sugar test will likely provide the diagnosis.

If the blood sugar is 200 or higher, then he has diabetes. If it is less than 100, he will need to consider other reasons for his symptoms. For blood sugar in the 101 to 199 zone, know that diabetes becomes more likely as the level gets closer to 200.

Less common causes for these symptoms:

• Diabetes insipidus
  The kidneys can't hold onto water because of abnormally low levels of anti-diuretic hormone.
• High blood calcium level (hypercalcemia)
  The kidney needs to put out a lot of extra water to deal with the extra calcium.
• Medications, mouth breathing or low saliva production can lead to a very dry mouth
  Dry mouth can lead to excessive fluid intake, which increases how much urine the kidneys make.
• Kidney disease
  In some types of kidney diseases, the kidneys can't hold on to water properly. Thirst is the way we prevent dehydration.
• An abnormal craving for water (called psychogenic polydipsia)
  People with this rare disorder drink huge amounts of water all the time. They actually can temporarily damage their kidneys.