Tell him the truth. I personally fdon't believe in heaven, but I told my son, when each animal has died (I live on a farm), that they die and go to heaven. In heaven they can play again and visit each other. He is now 4.5 years old and understands that death is perminent. That when something dies, it is not coming back. They may cry, but it is always better - especially at 4 yrs to tell the truth. then they will continue to ask for the pet. He may ask again several times over the next coming months, but you remind him -the pet is in heaven.
Also if Koko is still home (freezer) not buried, you may be able to let him say his goodbyes to koko. My son has even more recently gone with me when we had to put my dog to sleep. He said his goodbyes, and I told him it was the last time he would see Max and then I brought him in the back and we gave him a shot. When I came back to to the waiting area he asked to see Max, and I told him, Max was dead and as I remind him what we discussed earlier... he is fine knowinghis pets have died.