I think you could really do with someone having a look at whats going on in person. - It sounds like you have been trying the right things.
However my inclination is to advise you to put some further work into his recalls. Essentially your problem at the moment is that he wont recall to you with the distraction of having something in his mouth.
To try and avoid him getting this into a habit, I would focus far more on his recalls with other distractions. Work on calling him away from other dogs, interesting smells, carrion, food, animals - anything he may be distracted by.
An exercise you can try with the retrieve is to have him sitting in front of you. Place the dummy half way between you and the dog. Call him in encouraging him to pick up the dummy on the way to you.
You could try this on a long line to gently encourage him to come towards you.
If you can get him into you, dont be in a rush to take it from him if he is inclined to hold onto the dummy. Get down and fuss him while he is holding the dummy - let him forget that he has brought the dummy in before you take it from him, so you break the direct connection between him coming to you and having to give up his prize.
When the penny drops that the only way he gets fuss and another retrieve is by bringing it to you, the problem should greatly reduce.
Another possible strategy is to have a second ball or dummy in your hand which may encourage him to come in - although great caution here as it could encourage him to drop the orginal dummy.
A further strategy is to toughen up you approach to the recall, giving him a really firm, no nonsense order to come. Once again this has some risk with it and is very much dependent on the temperament of your dog and whats causing him to mess about.
General advise on gundog training is to greatly restrict the number of early retrieves and focus on obedience and steadiness. This is sound advice, but with some dogs there is always the risk that the owner is so worried about steadiness that the owner never allows the dog to develop a love of the whole retrieving process, which can cause its own problems.