I don't feed Iams - it is not a terribly good quality food but that is a personal choice from experience. I usually feed three to four cups of Canidae, which is a better quality food and my dogs stay in show condition. It is individual for the dog though - as long as the amount you are feeding keeps good weight to cover the dog, without making excess weight, that is the amount that particular dog should eat. It varies. If you can see the dogs ribs, he is too light - if you cannot feel the dogs ribs easily, he is too heavy. You may want to start having his thyroid panels checked through a veterinary university to get a baseline and make sure the doesn't have any thyroid issues (not uncommon in any breed and particularly with the addition of a non-quality diet). He may simply have a good or excessive appetite and trying to fill him up could be detrimental as it could put excess weight on him that can be dangerous to his health.
add: Free feeding a dog doesn't reduce any risk of bloat/torsion.