You can try chopping the cherry up into smaller pieces so that each one will give you 12 rewards.
If you give him a bowl full of different nuts/seeds, see what he always takes first.
Some birds just aren't food motivated. Once you get into training, if he likes it, you might even be able to treat him with his pellets. I also had a bird once who wasn't food motivated, so I clicker trained him with praise and excitement.
Keep trying -- I'm a huge believer in clicker training as a great way to keep your bird interested and to forge an even better bond!