Good advice by all, going back to basics seems to be a good start, 15 mins a day is all you need and and finishing on a good note is always important. Use the check cord and when he retrieves gentle reel him into you praising him all the way if you want him to sit then make him sit. Don't be in a hurry to take what ever is in his mouth with a young dog it is a "MINE!!" thing and he will want to keep it...;praise him pet him tell him he is the greatest dog ever and you will notice that he will gradually lower his head and relax.....take your thumb and roll his lower lip over his lower teeth and gentle pinch them together if he wont give and ask him to give....Hard mouths are a tough one and I think you have two options; lots of reps and patience or teaching your dog to force fetch which can be brutal and may be best done by a proffessional, but when done right will solve your problem.... best of luck,