Oh Barbra, everyone is right. There is no such thing as just a dog, and anyone who might think that has not yet had one they loved so much. When Gypsy had to be put to sleep, it took me a solid week to not cry all day.
I will suggest a great book, and if I had it in my posession I would send it to you, but my sister has it currently. It is one we have passed around, and it is perfect for children. It may help your son a little, especially as a springboard to talk it out. You can read it as much as he might want to until he feels better. It is called "Dog Heaven" by Cynthia Rylant. Full of great illustrations, and a sweet story to boot. It helped my kids, and my neice and perhaps it would help your son feel better too.