Porkchop the skinny cat

This isn't really a story, but an inquiry. My cat {Porkchop} vomits his food up all the time to the extent that he is loosing weight.

I've used Iams hairball formula thinking this is the best for him. I also have a kitten who gets kitten food.

Maybe he is eating that too and could it be upsetting his stomach? Or could there be some other health issue that is causing the weight loss?