What to do for cat who throws up constantly?

My 4-year-old cat has had a problem with frequent vomiting ever since I got him 2 years ago. This is way more often than a normal cat would vomit. It always happens within minutes of him eating. He can't keep down any canned food at all, and the mess from that wasn't too fun for me, so he is on dry kibble diet only. I use Iams hairball care formula, but I do not think this is from hairballs. (I never noticed any hair in the throw up.) It appears to be long clumps of undigested food. The most he can go is maybe 3 or 4 days without vomiting after eating and then it happens again. Remarkably, he shows no other signs of illness and manages to stay at about 10 pounds or so, which is normal weight for an adult cat. I even tried feeding him in very small increments several times a day and the results were, if he was going to vomit he would, even from less than 1/4-cup of dry food. Like I said, I have let it go on because he seems very normal otherwise and is not losing weight, but it is very frustrating to say the least! Could it be possibly be food allergies? Sorry for the long post!