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He will retrieve things, whether you want him to or not, it's best to start him on things you do want him to retrieve. A pair of vile stinky worn pair of rolled up socks is a good starting point.
'What's this then? (socks beneath nose, preferably not yours), 'fetch it'....
Ps: I would shorten that to 'drop', single word commands rule (unless you're my wife, who never uses fewer than 50).
And the next one when he retrieves things, which he will very soon (whether it's a ball, sock or half a rotting rabbit) is a word for 'kindly release it into my hand'. I use 'dead', which caused an unfortunate moment when after a game of ball with my lot, my granddaughter informed her teacher that she'd spent the weekend bringing back the dead.