Cat vomiting, no change in diet, any suggestions.

My wifes cat has been throwing up about 2-3 times a week. He is about 1 1/2 years old and we don't know what is going on.

Nothing has changed in his life or diet and he never goes outside or anything like that.

Well, what are you feeding him, and how much, how often? Is he an only cat? And is there anything else unusual, is he drinking allot, diarrhea, or anything like that? We would need some more info to give much of an opinion.