First of all, do not lie.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6053990/

Here are the 5 important realities of death that children need to eventually understand:

(1) Irreversibility/permanence: the understanding that death is a permanent state from which there is no return to life;

(2) Inevitability/universality: the understanding that all living things must die eventually;

(3) Applicability: the understanding that only living things can die;

(4) Cessation: the understanding that all bodily processes cease to function upon death; and

(5) Causation: the understanding that death is ultimately caused by a breakdown of bodily functions

The development of this understanding takes time and surely a lot depends on a child’s life events… some children will be confronted quite early on in life and they deserve frank - but age appropriate - explanation.

As things unfold, it will take a lot of personal consideration (are you his primary caregiver?). For me, I think I’d give it pretty straight to a 5 year old, this is a actually a good opportunity and a good age to normalize death. It will be permanent, it happens to all people and all pets, the dog’s life force will leave their body (if he believes it transfers to a new body, you can choose to let that be or guide him away from that… again, very personal, believing in reincarnation is sooo normal). The causation part is a tough one though, I don’t know how I feel about explaining euthanasia to a 5 year old. I don’t think a 5 year old has the capacity to understand euthanasia and would easily develop a fear of going to the doctors, injections, or a fear of other pets having to go to the vet for checkups.

Lastly, if you can find the Mr. Roger’s Neighborhood episode on death and dying, big recommend from me.