Hi chocolate,
I wanted to reply as the post was sad sorry to hear about the family dog.
I think what you've said sounds perfect. I have qualifications in working with children, and my own baby, and I think in your situation I would say exactly the same. You've explained what's happened, where he is now, given reassurance that he's not hurt anymore, and explained that sadly he cannot come back.
From experiencing this myself as a child, although I was around 3 so obviously had more of an understanding, my mum took me round the garden and I picked some flowers, and I wrote our cat a letter saying 'i will miss you' and she sent it to 'the special place' our cat was. I found this nice as it was a last something I could do, and I think I sent a blanket too, so the cat would be warm.
Obviously at 23 months your ds may not quite have the understanding as a 3 year old, but perhaps a picture or something?
I think my mum did this with me as I became anxious that our cat was feeling ok, and this offered a sort of closure?
It sounds like you've got it covered though. I hope it goes ok