Each individual deals with death in their own way. He is 5, I don't think that it's a big deal, that he did that with his alphabet soup, and he is not on his way to becoming a serial killer. At 5, they really are still discovering their emotions. He may be quite a stoic, person. I personally deal with stress and tragedy through laughter, I hate to see anyone sad around me.
You also have to consider, that in this day and age, we have become "Well, if it is missing, we can always replace it." It does not make it right, but this could also be where he is coming from. I would not push it with him. If he is not sad now, he may be a few days from now. If an animal is put to sleep, it is usually done away from the children, and as they have not witnessed this, it is sometimes hard for them to think anything other than, "Spot is away". I used to be a vet tech, and I saw this all the time. Some parents actually told their kids that the dog had gone off to live on a farm somewhere, where he could run around as much as he wanted.