Maybe you can follow through with the surprise but do it for all three...
You surprise the kids the night before or the morning of, and have a special card set aside with the dog's name on it for when he asks. Have DS open the card, and it's an invitation from Mickey Mouse for the dog to enjoy a week (or however long your stay is) with the family friend who will be watching the dog (maybe something like "we know Rover doesn't like Space Mountain and wants to stay home but would be lonely by himself, so boy have we got a great surprise for him instead!). Have a list in the card of fun things they'll do: play fetch, tug of war, have great meals, play with other dogs, go on walks, etc - you get the gist. And then have a special toy or new collar or something for the pet that your son can give him as a "we're all going on vacation" present.
If you can make it as though the dog is going on a fun vacation too, maybe he will feel better. And if the family friend will do so, have them text a picture a day of the dog so that DS can see how great they're doing.
Sounds like you have a very loving little boy!