So I'd start with restricting his space (start in the kitchen, hallway or somewhere similar) so he can't go rocking-horsing around with the article in his mouth, then calmly start working on him holding the dummy and increasing the duration of that. There's a million different ways of approaching this, depending on how quickly he picks it up (pun intended). Then I'd work on including the hand target. It can be a bit tricky for the dog to get this at first when they have the dummy in their mouths, so just as you'd shape the target to start off with, start with really low criteria of a slight look at your hand, then a slight turn of the head towards it etc. Eventually, you can work up to the dog following your hand with the dummy in his mouth.