Lots of time on the bench force fetch. You might have to go through the whole process again. If so, absolutely do not move onto the next step until the previous one is perfect. E-collar is your friend while teaching him to hold correctly. What kennel is he out of if you don’t mind my asking?
This. OP mentioned force fetch training already but what was desired was not fully understood by the dog. Go back to the beginning and do not move to the next step until the dog fully understands what is to be done. And, make sure whomever is doing the training is not glossing over steps or rushing through a set schedule.
Since you and the dog seem to have good rapport, this might work for you. I used it on my current DD puppy, and it fixed his hard mouth issue early in his fetch training. With the dog sitting, have him "fetch" your hand. May want to put a glove on first if you think he may bite down hard. If he won't open up for your hand, open his mouth and insert it. If he chomps down on the hand, feign excruciating pain and make noise like you're hurt. If he doesn't bite down, tell him good boy and give the release command (out, etc.). Do this until he readily takes your hand and holds it gently. Praise him to the skies when he does it well. Next, hold a dead bird in your hand and repeat. Hope it works for you and your dog.
This is a step in force fetch, part of the purpose is to prevent the dog from rolling things in their mouth and to further promote a soft mouth. You WILL want to wear a heavy leather work glove for this, an insulated leather work glove may not be a bad idea. (I wear a work glove through all the steps where the dog is expected to take an object from my hand to prevent injury to myself. I have a few scars teaching myself that lesson.) I implement it after the dog has learned to take training bucks from my hand. This is the step I would be especially concentrated on though the steps prior and after are also important.
