inadequate water intake; most-probably!
Drink atleast 4 liters of water a day.
All the best,
Parth Adhyaru
You don't say if he has type 2 diabetes or if you are wondering if that is what is causing his symptoms. Shakes can be caused by many things, but can be symptomatic of low blood sugar, especially if it happens when he is due or overdue to eat. This could indicate diabetes or hypoglycemia. I would see his doctor for bloodwork.
If he is a heavy drinker this could also be symptomatic of alcohol withdrawal.