Hope all goes well at the vets. As the others say, it means real vigilance to ensure the cat is never in the same room with the hamster, either in or out of the cage. Just the scent of the cat nearby will cause the hamster stress. Poor little guy, I hope he is ok. He might need a couple of quiet days in his cage now, and lots of nice extras to distract him from his trauma (you could hide a treat or two in his substrate or in different parts of his cage and/or a new chew stick or extra special treat like a very small piece of cheese
This may set taming back a bit if he is nervous being out of the cage or handled now, but taking it slowly and gradually should help rebuild his confidence.