How's he bred, how old is he and have you had him since he was a puppy? Keep on prising his mouth open with the command LEAVE! Get back to basic retrieving training with a dummy. Get him to fetch it and when he gives you it/you take it off him say LEAVE! He'll soon associate this command with giving things up. Good luck mate.
hes 5/8th grey hound greyhound or there abouts and hes 4 ive had him six months hes been in the family . ive put alot of work in with him hes retrieving with a dummy and everything else i throw for him he used to bark at rabbits when chasing them i got him out of tha