So sorry to read this, I do feel for you.
If I can suggest a piece of practical advice when the time comes -
if the vet comes to you, say your goodbyes to him, and then all leave the room and leave him with the vet. Ask the vet to take him out of the house as quietly as possible, and make sure you are in a room with no view of the vet going out with him, or if that is not possible, draw the curtains in advance.
That way you won't suffer seeing him going on his way to peace and freedom from pain.
A x