I used to have a cat named Peanut who was best friends with our dog Cocoa – he was practically in love with him for all of his 9–10 years of life. When Cocoa died, Peanut got very depressed and was definitely not himself for a period of a few weeks. During that time he also developed eating problems, and in this case it turned out my cat was diabetic. He probably had been suffering for some time, but the death of my dog seemed to be so traumatic for him that it made his symptoms more apparent.
So, yes, I do believe it’s possible for cats to get depressed. However, I don’t think these feelings will last very long – your cat will probably get over his depression in a few more weeks. In that time, be very careful about his health, which (from my experience) can be very vulnerable during periods of mourning. Try to feed him his all-time favorite foods. High-protein foods like tuna are good, or even kitten food, which I think has more fats than adult cat food. If you have to, take him to the vet.
Other than that, just give him a lot of attention and make sure he feels loved. I hope Pixie comes back, but if he doesn’t, you and Samson have to be there for each other.