What Causes Excessive Appetite Along With Sweating In A Toddler?

My 3 year old son is overweight and never seems to be full. He drinks excessively - about 6x 200ml water per day and urinates frequently. He sweats a lot too, often when exercising but sometimes in an air conditioned room with no physical exertion. His child care workers have suggested I get him tested for diabetes. His breath and urine do not smell as you may expect from a child with diabetes. Another thing I have noticed is that his stools, although good in texture, have an odd ribbon type shape to them. Any ideas if this could be linked or signify another issue?