A. - well thanks for making my mascara run! I am a blubbering mess! Oh, I am so very, very sorry. I cannot even begin to express my sympathies. I come from a very big pet-loving family - we're all the rescuers. And I have had to put way too many pets to sleep. It's never easy, and through that grief, you also need to help your son.
I don't know if you know the poem Rainbow Bridge - you can find it at www.petloss.com. This is a great poem. Maybe you can read it with your son. Frame it with pictures of your babies.
Perhaps building a memorial in your yard - have your son design it and then help him build it.
Consider having a memorial service to honor both pets. It doesn't have to be elaborate, but just as we formalize saying goodbye to humans, we should also do the same for our four legged family members.
When I was a child and I had to put my first cat to sleep, my mother bought me a book called "The 10 good things about Barney" - which was about a cat that had died and how the little boy coped with it. I still have the book - it meant that much to me. And everytime I have to put a pet to sleep, I dig that book out and read it. Maybe go to Barnes and Noble online and find a good book about the death of pets and go and buy it for your son.
Here I go crying again. I am so, so sorry for your losses. I hope that maybe I have given you a few ideas that might help your family cope with your loss. You will be in my prayers.
S.