My parents lost a hamster & 2 dogs during the time my daughter was about your sons age. We didn't address the idea of death with her. We didn't tell her that they died. When she asked where they were, we just said they went to live somewhere else. She was ok with that answer & just moved on.
This may not work out as easily with your son since these were pets in his own house (not grandparents' house). My parents still had 2 other dogs, so there were still dogs to play with. The 2 we lost were very old & didn't play with her much anyways.
If your son was really close to these pets, you may want to try a book written for this. I don't know of any, but I'm sure other moms do! Or you could try to simplify your family's beliefs about death. Tell him that they went to live in a special place called Heaven, etc.
Sorry for your loss. It's so hard to lose pets!