how much food?

I have a question about how much to feed. I have a 9 month old pup that weighs in right at 70 lbs. He is not skinny and he is not fat. Actually I think his weight is just about right. You can almost see his ribs. but not quite. So here is my question. I only feed him two cups of food a day. (one morning, one evening) . He is on euk puppy food. He gets alot of work on land and water and is always hungry. Could two cups be enough to sustain him? I dont want to put weight on him too fast but I know he also needs the nutrition. It just dosnt seem like much food.