How thirsty is thirsty? I remember reading about one woman who has two kids with type I, and she said that she knew as soon as she walked into her son's room and saw water bottles lining the wall.
There's a lot of reasons for increased thirst, including something as simple as a change in the environment. However, if you're concerned it's not a bad idea to get him checked out.
Other symptoms include:
-Unusual weight loss or gain
-Extreme thirst
-Increased urination
-Fatigue
-Nauseau
-Dry Mouth
-Cuts that are slow to heal.
I have never heard of leg pains as a symptom- chances are he is experiencing growing pains. I know that whenver I hit a growth spurt my legs would hurt.
Bottom line, if you are worried, get it checked out. Uncontrolled blood sugar can do a lot of damage that is irreversible. The thirst is because high blood sugar actually thickens the blood, and the body is trying to thin it back out. This thickness is what damages the blood vessels, causing peripheral neuropathy in the feet (the reason so many diabetics end up with amputations.) If it is diabetes, the sooner you catch it, the less likely he is to suffer complications.
Good luck, I hope it turns out to be nothing.