How to put a child a diet?

i have a brother that weighs 75 pounds and he is only 6. his doctor put him on a strict diet but when ever we turn our backs on him he gobbles up food with us not knowing. when we ask him if he eat food he lies till we put something that he loves on the line then he pleads guilty. he is part of a family of 6 and we are deeply worried about him.he is also on the edge to get diabetes. i need help desperately!