Then let him sit* in your lap until he's sufficently calmed down enough to go hide under the bed for the next 15 minutes.
To get your cat back to normal, let him go collarless for the rest of the afternoon and for good measure, fill his favorite stuffed frog (courtesy of Julie) with a pinch of Cosmic Catnip. He'll be good as new before you know it.
*Note, if you will, the way his feet are curled up in tense anticipation of the bell coming back.