I'd take him immediately off of the Iams and give him something
like Innova canned (this is great for kittens - get the red can).  
Has he had his first vet visit yet?  He needs to be checked for
worms and get some vaccinations if he hasn't already.  And
neutering.  :)  Worms and other parisites can cause awful smelling
poop.  That said, all poop stinks.  ;)  How old is he?

Hello Dixiegrl.

I adapted an older cat and his poop smelled really strong and bad.

I think this problem is caused by the wrong type of bacteria in the
gut.

I fed my cat Yogurt and the problem cleared in a short time.

Try it. I am sure this will fix the problem in most cases.

If your cat never had yogurt he might not want to eat it. So mix the
yogurt with some people food. Tuna fish in water is good.

When you get the yogurt read the label and make sure it contains
*Active Cultures*  These active cultures will reestablish the
beneficial Bacteria in the gut and replace the bad bacteria that is
causing the problem.   This works for People to.   :-)

Also get the Plain Yogurt for your cat. That means NO fruit and/or
junk additives or flavors.

Good Bacteria in the gut is necessary for health. Many of the B-
Vitamins that you need are made by these Good Bacteria in the gut.

Let us know when the cat is all better so other people can try it.