The back garden is the best place to do this. Somewhere safe and secure with no distractions.
Let your dog scamper around and play, maybe throw a dummy or two for him to fetch.
Taking the cold game, rabbit or whatever it is, drop it onto the floor and encourage your spaniel to investigate.
The chances are that he will come up and sniff at the game and perhaps mouth it. Encourage him to investigate and, if he picks it up, brilliant, encourage him to bring it to you.
There is also the possibility that he may back away, tail between legs, maybe stalking the game as he builds up courage. He is simply uncertain and you should encourage him to sniff it, maybe hold it yourself and show it to him.
You want him to investigate and not be afraid.