Purina One isn't actually that great of a cat food, especially for the price. A cat food that isn't made with real meats would be garbage... what you want is one that has fewer fillers. Your formula:
"Chicken, chicken by-product meal, corn gluten meal, brewers rice, soy flour, animal fat preserved with mixed-tocopherols (form of Vitamin E), fish meal (source of DHA), whole grain corn, poultry by-product meal,non-fat yogurt, wheat gluten, brewers dried yeast, phosphoric acid, caramel color, animal digest, salt" ..(vitamins and such)
There are a number of things that the best cat food would not have. Cats have zero need for grains, especially corn and soy. Also, by-products are just about the nastiest thing on earth (well, aside from animal digest which is also in this food). It is good in that its first ingredient is a main meat... and then it pretty much goes downhill. They've done a whole lot of really good marketing to convince you that its worth paying so much for. It isn't.
You'd be amazed the things you could learn about feline nutrition from this site.
Anyway. He was probably dewormed in the shelter, I hope!
I do hope you can get your boy to clean his rear!