D.W.
I think you should just talk to him about Patchouly. Kids that young don't really understand death and some are more sensitive to it.
I was 6 when my parents had to put my dog to sleep because of some illness. I remember being 9 and still grieving for my dog. She was my first pet and I'm an only child so I felt even more close to her because I didn't have any other siblings. My parents let me talk about her whenever I wanted. They got me a cat for Christmas when I was 9 and somehow that helped me to forget that I missed her.
I also read the Ten Good Things About Barney about this same time. It was very good and I also highly recommend it for your son.