At his age, I would distract him by getting a new puppy. This doesn’t mean you can’t still talk about the old dog—just reassure him that Fido is in doggy heaven, and this new puppy is his, now. It worked for me when our kids were small.
Excellent advice. Children must learn that their pets do die but to explain it so beautifully is what is important. Remember to tell the child we NEVER REPLACE the last pet. We will always remember him. We just give a new pet a brand new family to love and for us to love and take care of each other. They will understand eventually.