I would say the same..
Have two balls, throw one ball, he will chase it, call him to you, he will probably come near to you but not close enough for you to take it,
Pull another ball out of your pocket, and say " whats this", he should drop the ball in his mouth because that will be dead prey to him now.
then you could claim the ball he has dropped with a leave etc, once he is consistenly dropping the ball then add a key word to it.
Hope this helps, good luck
xx

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Sarah xxx