Mitty,
Here is another of Blimps aphorisms ... "be consistent but not predictable".
You chuck out a dummy and the dog thinks "I'm going to be sent for that, perhaps I should go right now and save a bit of time". He's right because every time you throw a dummy, guess what? He's sent for it. So you need to break the links in his mind that give rise to expectation of a retrieve.
You can do this in lots of ways and the first is to pick up a high percentage of the dummies yourself leaving the dog at sit. Then instead of casting him off, put him at heel and walk away from the bird. Then throw another dummy and .... walk away from that one too. Throw a tennis ball and cast him off, but stop him immediately on the whistle and send him for the first mark. Then stop him, and call him in, and cast to an un-cued blind. You know the dog better than we do and you can judge how far to take these remediation measures, but from now on until you are confident he's getting the message, NEVER send him to the expected bird. I used to do this with Springers and make it all a big game, mixing up literally dozens of commands, verbal, whistle, and hand in any one short session, but ALWAYS being one step in front of Fido. As regards the timing of the command, don't just count up to five, you need to put his reward (the retrieve) on a variable schedule, and you should be absolutely unpredictable; anywhere from one second to three minutes.
Many ladies of my acquaintance know the secret of relationships ..."keep 'em guessing."
Eug