Yeah, Kitten Chow has really crappy ingredients, especially the dry. It may or may not be related to his skinniness, but it would most certainly be better for him if you fed quality wet food only. Here is some eye-opening reading on feline nutrition: www.catinfo.org
And there is no way a vet can tell if a cat has tapeworm just by feeling his belly. Me thinks she was trying to sell you a dewormer.