"I grabbed him by the scruff of the neck and said "BAD CAT!!"
Grabbing him by the scuff of the neck and yelling at him won't do any good. Is he neutered??? If not, he needs to be.

I would also feed him nothing but cat food (no table scraps, no treats). His colitis is obviously bothering him and you getting angry at him for something HE has no control over isn't going to help. It's only going to make him feel worse.