If your male is "gulping down everything", he shouldn't be free fed. The only dogs who should be free fed are the ones who barely eat each time they decide to.
Could be the food. I noticed when we switched to the Solid Gold, Boo stopped having trouble going. He was abused too, which had an effect on him. Iams has stool hardeners in it which would probably cause constipation because the longer it stays in there, the harder it will be, right? And the big food companies think that harder means healthy. (So even when your dog is sick, you don't know until it's really bad...)