Iams is not a great food, so that might be part of it. Switching abruptly could give him diarrhea though, so consider that too. Maybe buy a higher quality food and put it in a separate dish so the hedgie can choose?
If you can get some small animal benebac (from a pet store) or acidophilus (from the grocery store) and sprinkle that on the food, that may help.
You could also get a variety of baby foods to offer and see if your hedgie likes any of them, then mix the benebac or acidophilus powder into that. Sweet potato and meat flavors are popular, my hedgehog loves peas and carrots.
Make sure he's getting water too, give it to him with an eyedropper if you think he's getting dehydrated.
If you don't see poop soon (check everywhere in the cage, they are good poop-hiders) take the hedgie to the vet, home treatments for constipation are pretty limited and he needs to keep things moving through so he can eat and grow!