I am very sorry for the unexpected loss of your cat.
Fortunately, our two cats and two dogs whom our 2 1/2 year old adores are still very much alive, but we did just recently go through the death of our fish. Although we'd had the fish for over a year, I didn't think our daughter would notice its absence, but of course she did, within a day. I told her Artie was gone from the house but he was still with us in way. I think heaven is too abstract an idea for a toddler, so I took her outside to the spot that I buried the fish (yes, I buried the fish. I am insanely sentimental). If you haven't buried Shadow you could pretend if it isn't too late or doing something like a memorial. I explained to my daughter that Artie had gotten sick and old and died and that I had buried him in the earth where he had come from. Now instead of being with us in a way where we could see him and feed him, he was in the earth and all around us. I let her plant a little flower on his grave so she could see how Artie would now help other things grow and live. It seemed to work well, although as I said this is just a fish and not one of the pets she interacts with all day, so it may be different for you and your son. Occasionally when my daughter is in her high chair she points to the grave spot out the window and says, "There's Artie and his flower, but he isn't with us anymore." She seems to get it and isn't emotional about it anymore.
Best of luck. And again, condolences on Shadow.