You can try to start clicker training or target training any time, but you do need to find a treat that he loves in order for it to really make an impression...And whatever treat you use for any type of training should be his all-time favorite treat, and then you should only give him that treat as a training reward and no other time. That way he knows that in order to get his favorite treat he has to listen to you and progress with the training.
Have you tried giving him different types of unsalted nuts? My Quaker absolutely loves pistachio nuts, those are her extra-special treat that she only gets for training and as special rewards. The other nuts that they always seem to love are pine nuts, Brazil nuts, Walnuts, Almonds, and even raw, unsalted sunflower seeds out of the shell work great for training...Of course any nuts you give him must be unsalted, and for a Quaker they need to be out of the shell, as they can't really crack shells very well. As far as the sunflower seeds, my birds don't get them any other time but as an occasional treat (their seed-mix does not contain them), and I simply buy a container of the raw, unsalted, out of shell sunflower seeds as training treats for both my Quaker and my Green Cheek, they love them. i don't like giving fresh fruit as a "treat", it would work, but fruit needs to be a very small part of their diet, at least fresh fruit does...If your Quaker doesn't respond to any nuts (which I doubt, the pistachio's and the pine nuts usually work on all parrots, lol) you could also try some dried/dehydrated fruit, I was surprised not long ago to find that my local Walmart has a massive selection of all-natural, organic dried fruit snacks with no added sugar or preservatives. They have a big rack that is nothing but organic and all-natural nuts and dried fruit snacks of all kinds located right next to or right in the fruit/veggie produce section. They have all kinds of organic, dried apples slices, peaches, apricots, coconut (this is a good one too that they usually love), strawberries, blueberries, raspberries, blackberries, bananas (another one that they love, my Senegal goes nuts for dried banana chips as a treat), mango, guava, passion fruit, star fruit, papaya, etc. And they're much cheaper than Wegmans, which is where I used to buy this kind of stuff because I didn't know Walmart even sold any.
So it's all about just trying different training treats and seeing what he likes...you definitely do not want to use his regular pellets as a treat, that would only be confusing to him and really not a big deal to him either, as he eats them every day anyway. I'd start with trying-out different unsalted, unshelled nuts, as they are quicker and easier for them to eat on-the-spot for training, and you can keep them in your pocket to reward him on-the-spot, or to take them with you when you take him out and about in the future, say at a Vet appointment or on a long car ride.
One more little tip about using nuts as a treat, for smaller parrots like Quakers, Green Cheeks, Cockatiels, etc., don't give them an entire nut as a single treat, only do that at the very end of your training session to tell them "Good Job!" and then give him a whole nut to eat. Otherwise, if you give a Quaker an entire pistachio nut or an entire raw, unshelled Brazil nut or Filbert nut, it's going to take him 5-10 minutes to eat it, lol. You can do this with larger parrots obviously, even my Senegal can crack and devour an entire, in-shell Brazil nut, Walnut, Almond, etc. in a couple of seconds. But with the smaller parrots, simply break off little, tiny pieces of the nut and after each time he does what you ask of him and you hit your clicker, then just give him a little piece of a pistachio, pine nut, etc., You can however give a Quaker an entire raw, unshelled, unsalted sunflower seed, those they can eat in a few seconds. The nuts for whatever reason they tend to hold in their foot and chew on for quite a while. So just give him a tiny, little piece of nut, that way he'll be able to eat it quickly and you can continue training. This will also help to keep him focused, so he doesn't lose interest and want to just sit there and chow-down on nuts, lol.
