Re: Hold things in his mouth CALMLY!
I can offer advice as we are working the the dumbell retrieve. According to my instructor this is one of the more difficult things to teach your dog, if it does not come naturally.
I used a clicker, and I started working on it at least 3 weeks before the start of the class.
I only use the dumbell for training and do not let her have it as a play toy. I try to get her excited when I bring it out.
I started off by holding the dumbell while I was sitting, click and treat everytime she looked at it. Move to click and treat when she touched it with her nose. Move to click and treat when she placed her mouth on it. Move to click and treat when she held it in her mouth. Clicking ends the behaviour so once the dog is holding it and you click they drop it. Gradually work on getting the dog to hold it longer before your click.
Move the dumbell lower and lower to the floor, so your dog is reaching down to grab it. Add commands, take it when she reaches for it, hold it when she has it in her mouth, and give when you go to take it.
If your goal is to do a formal dumbell retrieve DO NOT let the dumbell fall to the floor. You want the dog to pick it up, bring it to you and wait till you ask her to give.
Some people wrap their dumbell with material, if using plastic, so the dog does not roll it. The owner of the golden wrapped a leather glove around his dumbell as his dog loved to play with it. It made the dumbell thicker, but he is now taking it. Now he has to take it off and work on him taking it without any wrapping.
Dakota would roll it and flip in her mouth when she brought it back to me. To stop that work on the hold, tap the dumbell while it is in her mouth wich causes her to grip it harder. If they are still spinning it you can gently hold their muzzle. Before you do that with an object in their mouth you should get them used to their muzzle being touched and then held shut. Dakota does not like this much.
Hope this helped.