Has he been in a lot of heat, so that thirst would be natural? And - yes - it sure could be fun to ask - but - if that were all it was, he wouldn't drink it all after he asked for it (well, mine wouldn't have...they'd have had a sip and then tossed tossed it aside, satisfied that they asked and received).
At just about that age, my son had an excessive thirst and, with a family history of diabetes, the doctor wanted to have him tested for diabetes. He did not have it (and, now at age 39 - still does not have diabetes). But - it is a symptom...If the heat doesn't explain it and it doesn't go away it a while - or if you see other symptoms (weight loss in a child is a pretty dramatic symptom) - I'd ask the doctor what he/she thinks...as for limiting the liquid intake, I'll leave that to others, but young children seldom need anything limited - except for sweets and fats (the juice should be limited - substitute water - if he's still thirsty - I'd give it to him.)
Good luck!