Reddit reviews The Goodbye Book

A few picture books you might read together -- Lifetimes, which gives a comforting non-religious perspective on the cycle of life and death; Goodbye Mousie, which features a boy about your son's age whose pet mouse has died; and Todd Parr's The Goodbye Book, which is about saying goodbye to a goldfish (from the perspective of another goldfish). Just keep reassuring him that he won't die for a VERY long time, and that death is what happens when animals and people get very old and sick -- it wouldn't be much fun to stay like that forever, so when they get too old and sick to be happy anymore, they stop being alive, just kind of run out of steam. (And hopefully it will be a while before he has to deal with death in a less ideal context...)

Since he's already been exposed to the idea of heaven from your mom, you can tell him that some people believe animals and people go to another world after they die; it makes some people happy to imagine that place and tell stories about it. You can say no one knows for sure what happens after we die, besides our bodies turning into Earth again, so it's okay for everyone to have their own ideas about those things. Personally I view heaven as a comforting story rather than a literal place -- and I think it wouldn't be confusing or a cop-out to describe it in those terms. You can also talk about how even after people and animals die, we keep remembering them, so they're always with us, in a way. The book Always and Forever does a good job illustrating that idea.

It's pretty normal for kids to be freaked out when they first encounter the idea of death; just keep validating his feelings and talking through them, and he'll probably work through it soon.

DH was widowed and a single dad to a two year old before we met. His recommendations are The Invisible String and The Goodbye Book

I'm so sorry for your loss.

I think not sugar-coating it is best. This age your son is able to understand things better than you might think. One other thing I would add to that the previous poster said, is tell him that it is okay to be sad, that you and his dad are sad, too. And that he will probably always be a little sad but the feeling will get easier after a while.

These are 3 books I have on hand. My daughter was very broken up when our dog died, and her Grandmother is failing in health as well:

https://www.amazon.com/gp/product/0316404977/ref=oh_aui_search_asin_title?ie=UTF8&psc=1

https://www.amazon.com/gp/product/0553344021/ref=oh_aui_search_asin_title?ie=UTF8&psc=1

https://www.amazon.com/gp/product/0552567655/ref=oh_aui_search_asin_title?ie=UTF8&psc=1

This book was helpful to us in a similar situation.

The Goodbye Book https://www.amazon.com/dp/0316404977/ref=cm_sw_r_cp_api_i_eic1DbR6602HV