How to get my old cat to get used to the new kitten?

Butters, is our 4 year old male cat. He is pampered 24/7 and gets so much love and attention. We helped my grandma out by taking in Theo, a 8 week old little boy kitten. Butters always hisses, growls, and even smacks Theo. Butters barley even comes around me anymore. But occasionally Theo and butters DO play. But then Butters starts getting grouchy. Any suggestions how to help this?