Playing ‘keep away’ is a really common problem especially in young dogs just becoming more independent.
Assuming that you have a rock solid recall in the dog when he is not carrying the dummy, the problem is caused either because he wants to engage you in a game or because he is possessive of the dummy and wants to keep it for himself.
A dog which just wants you to play will come around and start delivering the dummy if you take him to a wide open space and use the ‘walk away’ technique*
A dog which wants to possess the dummy for himself is a different challenge and the quickest way to over come this is with rewards, provided you use the correct technique
Like many training problems you usually have two basic options - reward correct behaviour, or punish incorrect behaviour (or a combination of both). Both have their difficulties where retrieving is concerned
If you punish a dog (scold him, or reel him in on a training lead for example) there is a risk he will drop the dummy en route. Traditionalists will tell you that if you reward him for giving you the dummy, then he will also spit out the dummy to get the reward. Which makes it seem as though you can’t win!
However, this is true only if you do not use the correct procedure for training retrieves using rewards (food or toy) and very few gundog trainers currently use these methods. If you follow the correct reward based procedure*, you can train a neat reward based retrieve /delivery which completely cures the ‘keep away’ problem for good. It will however take several weeks and is not a quick fix.
There are lots of variations on a theme where retrieving is concerned, and most experienced trainers have their own ‘favourite method’ for curing ‘keep away’ Are you currently having lessons?
*If you like I can pm you links to descriptions of the various methods available, but there is nothing quite like first hand advice by someone with experience and access to your dog.