I've cared for dogs over 5-6 month periods and given them back to their original owners before. Even though dogs create an emotional attachment to their owners, I think they are smart enough to realize when someone else is feeding you, walking you and taking care of you, then that person becomes their new (or original) owner.
Eating Issues / Concerns
If I was in your uncle's position I would just keep an eye out for eating problems. I'm sure the dog won't go more than a couple of days without eating, and while it breaks our heart and we worry, we must remember that they are dogs. They don't think or feel like humans do; we must respect that and we should be in enough control to do what's best for the dog, by not spoiling it beyond control and remembering he's a dog, as much as we love 'em and want to hug 'em!
Try to keep a strict feeding regimen (schedule). If your dog has the bowl and doesn't want to eat because he's in a strange place or you think it's depressed, then take away the bowl after a couple of minutes of trying and don't feed him again until the next feeding time comes around. Don't yell at him, don't put him down, just be patient and serve the plate at the right time of day every day. Always make sure there's water for him to drink.
I am 99% sure the dog will recognize your actions and start eating sooner than later; a few days with no food is "not a big deal". I've successfully trained my dogs to eat at the same time even when sometimes one of them was too picky to eat when I served the bowl and said "eat!".
Tip: I used to put a little broth from chicken or beef soup just to moisten the dog food and entice an appetite with greater success. I only did it a few times or when it was available and I felt like it. Just a little splash of broth will do enough to make the Alpo taste much better!
Re-homing the Dog
When we have pets, such as dogs, I believe we need to establish ourselves as leaders to them. Dogs naturally expect to follow or lead, and in our daily lives, it's much better that we lead and not give them the responsibility.
You don't need to ask the dog permission or see if he wants to leave with someone or not; you simply do it (without any physical abuse, of course). Your uncle needs to learn to be patient and willing to accept that the dog will take a little bit of time to recognize him as his new master. This is completely normal, in my opinion, and the more he establishes some rules and limitations with the dog, the sooner the dog will realize his position in the pack with his new leader (your uncle).
In one of the comments, @JoshDM said you should avoid being present in the dog's vicinity for multiple weeks. I completely agree with him and think that it's a good thing for the dog to get used to the fact he's going to be in a new home and it's OK for you to come, and visit, and leave.
Tip: When you come to visit, don't just run straight into saying hello, making the dog pee itself with happiness. It's s not healthy for him to get over-excited, and then sad again when it comes time for you to leave. There's always a time to play with your dog and be at ease, but we as humans tend to break that and just give them a lot of love and affection because it's super hard to resist not doing when you love your pets.
Walking with the Dog - Creating the Ultimate Bond
This is something I really agree with "The Dog Whisperer" (Cesar Millan); simply walking with your dog creates a very cool bond and respect for each other. This is how the dog starts to acknowledge you more as a pack leader; you're out letting him explore with you, while you lead the way, the pace, and the time.
Hopefully during it's time you, you've taken the time to teach the dog to walk properly on the leash and try to ignore cars, other people, and the daily distractions of life in the city. Going out for a walk with the dog should help strengthen the bond between your uncle and his dog.
Tip: In your mind you should also accept this if it's what's in the best interest of everyone. Accepting this change will make your energy transfer positive feelings and reassure the dog that he's going where he belongs and will be safe.
Hope this helps, Cheers.