If your rottie has something in his mouth that you don't want him to have, you must take it from him. But grabbing something out of a big dog's mouth can be a risk, especially if you didn't raise him from puppyhood. A less threatening approach is to use the command “Trade.” “Trade” tells the dog that you will give him something very yummy if he lets go of whatever he has.
The only way to teach “Trade” is to prepare for it. You first must find out what is absolutely irresistible to your rottie and be very stingy with it. Liver, fish, cat treats, bits of hardboiled egg, peanut butter, and certain types of sandwich meats, like liverwurst, may be good choices.
When you see your rottie chewing a toy, go up to him and hold out the delectable treat and say “Trade!” If he catches a whiff of his favorite food, he may drop the toy (click and treat, if you're using the clicker). At the same time you reach for the toy, give him the treat, so he is occupied. Hold the toy for a few seconds and then give it back. Praise him!
This does two things. First, it teaches your rottie that you'll give him something tasty if he lets you take it. Second, if the item is acceptable, you'll give it back to him.
What if your rottweiler won't leave it or trade?
In rare circumstances, you may have to insist that you take the item away or keep your rottie away from it. Try anything you can to break his attention away from the interesting thing (with food, commands, or even correction, if necessary). Clip a leash on his collar and lead him away.