I wouldn't worry so much. It's not about leadership. It's a defense instinct that some dogs sometimes have more abundantly than others. Even in nature, heirarcy has nothing to do with an animal defending his food, even from a more dominant animal. If he just said, "Sure, go ahead and take my bone," he'd starve to death and be unable to breed, reproduce and the species would die out. That said. It is not abnormal behavior to feel defensive over his stuff. Repeatedly taking things from dogs can cause this. Some dogs don't care. Some do.
What we have to do, as humans living with dogs is teach them a new way to feel about us getting around their stuff. First, I recommend putting away the rawhides. First, because rawhides are dangerous. Small pieces can break off and lodge in their intestines, causing a blockage and death. Second, the counter-conditioning exercise I will explain needs to be done, starting out with lower value possessions.
Put all toys and things he likes away except when you're about to start one of your many practice sessions each day. Find a toy, ball or other chew bone that he likes, but that he doesn't go "ape" over, as he does the rawhide. As Redyre said, you'll be trading him for a treat. Always use a higher value treat to trade for his less valuable toy or chewy. You can use small pieces of hot dog, cheese, chicken...something yummy.
This is going to be a game, so get your happy, playful voice ready and sit down on the floor with him. At the very first, you'll show him what you've got in your hand while he has his toy in his mouth. Bring your treat close to him and he'll drop his toy to get the treat. Pick up the toy simultaneousy as he takes the treat. Praise and give him back the toy. Make a fuss over him. Repeat.
Next, you're going to add a cue word, like "give" for when he is to give you the toy and "take," when he is to take the toy back again. When he starts to look like he's getting into the game and enjoying dropping the toy for the treat, you will stop showing him the treat ahead of time. With your friendly voice, say, "give" while you hold your hand, cupped under his chin. He should let go and you quickly give him a tasty treat which you'll have in your pocket or on a table right next to you. Praise and give the toy back, saying, "take."
Next, you can add some fun to the game and toss the toy 2 or 3 feet away and encourage him to go get it and bring it back to you. "Give"...he gives. You treat and praise. And "take." He gets the toy back again. You can mix it up...sometimes do it like that, sometimes toss for a little retrieve game.
When he is really getting onto this, and realizes that he not only gets the toy back, but a treat, praise and a game, he will not worry about you taking his toy.
Now, it's time to move up the heirarchy of value as far as toys, balls or chewys are concerned. But still.....do not use a very high value item. Move up gradually and play the same game with each toy....several times a day for a few days. Don't move up to a higher value toy until he is very, very comfortable with the previous one.
Soon, you'll be able to trade and not give the toy back every time. Remember to trade up....that is, that what you give him is better than what he has. Lots of praise and change the subject if you're not giving it back again. Continue some other kind of fun.
To make a long story short, you're proving to him that not only are you trustworthy to take his stuff, but you're totally wonderful because you give him better things in exchange. It's a win, win situation.
More obedience practice where he complies (as he learns things) and he ends up with great treats, praise, games. It's all good. That will help too. The more he trusts you, the better.