I got DD to give up hers at 14 months by waiting until the one and only dummy she used fell apart (I refused to buy more) and I left the handle pat of it lying around for her to find. She very quickly figured out it wasn't the same, threw it on the ground and never wanted one since. I thought I was going to have a hard time because she had always used one to fall asleep, but no she was fine.
You could always do the dummy fairy. Dummy fairy comes for all the dummies and leaves a nice present in return, but only if ALL his dummies go and they can't come back. I have heard of people tying them to helium balloons so they float away to the fairy etc. Makes it his decision then.