He may simply be dehydrated. Does he have any other symptoms like trouble going to the bathroom or diarrhea? He may also be drinking more simply because he's growing. If you're really concerned or there are other symptoms, contact your doctor.
You might want to rule out any physical problems, such as juvenile diabetes, which can manifest by extreme thirst. Otherwise, make sure he gets enough solid food along with liquids.
It's okay as long as he's also urinating as well. Observe it and if your child looks like he is always thirsty, I advise you to have him checked by his pediatrician.
If he is drinking a lot due to playing outside a lot when it's hot or some other reasonable explanation than you don't need to worry. If that is not the case then it could be a sign of juvenile diabetes and it is very important to have him checked by a doctor.
