When he vomits, is it near food time? Is the food basically undigested or mostly digested? That can tell you a lot about cause.
I'm not overly impressed with Iams as a food, especially since it got bought out by Proctor & Gamble a few years back. And 2 of the top 4 ingredients are corn by-products in the hairball formula, which isn't all that great for cats. It doesn't look like they have the sensitive stomach formula on their website, though, so I can't check the ingredients on that.
Okay, in double checking the ingredients, they put inositol in their food? Why? That makes no sense to me, and inositol can cause nausea, though it's hard to tell if there's enough of it in the food to cause an adverse reaction, but that might be a part of the issue right there.
Have you always had him on Iams? Have you tried other foods to see if something else would cause less puking?
Also, just because he's not bringing up hairballs doesn't mean he doesn't have them, especially in a cat as fluffy as a Maine Coon! Try offering him some canned pumpkin (not the pie filling kind, just plain) and see if he'll eat it. Some cats will take it plain. Otherwise, mixing it with some canned food generally makes it go right down. This will up his fiber intake, and help pass out any hairballs that may be blocking up the works just enough to cause vomiting.
