I would suggest sitting him down, giving him the dummy ie dont do a retrieve as such, then backing off only a yard or so. Call him to you and being careful not to stare him in the eyes put one hand gently under his chin and wait a few seconds. Give him your release command dead or whatever you use and gently take dummy out of his mouth. Wait a few seconds and repeat say twice more. Repeat daily. If he still manages to lower his head then do the same but without backing up the yard then if you get progress you can then move back a bit and build it up. Progress eventually into a short retrieve. Let me know how you get on. Good luck.