Hi Vicky,
A raging thirst is a very common symptom of diabetes. Effectively you have a blood sugar level that is too high and the excess sugar causes dehydration, which makes you want to constantly drink. You mentioned that your dad likes beer, but that is extremely high in sugar so that is the one thing he mustn't drink. It will just raise his blood sugar even higher and make him even thirstier. The thirst will get less as he gets his blood sugars under control.
His medication is most probably metformin as that is the standard treatment for diabetes. Unfortunately stomach upsets are a side effect, although they often become less severe as the stomach becomes more accustomed to the medication. He needs to ensure that he takes his tablets with food, not before or after food, so take them in the middle of a meal. That generally makes them more easily digested. If the stomach problems just don't wear off then there is a slow-release version called Glucophage SR that is much kinder on the stomach. Doctors generally don't prescribe this unless they have to because it is much more expensive than standard metformin.
Finally, you asked whether you will get it. There is s genetic link but also there are many people who have diabetes today who have no previous history of it in their family (e.g. me) and others who have diabetes but their children never develop it. So the answer to your question is "possibly" but not "definitely".