Cockers are certainly not untrainable.
You just have to find the right motivation.
We do obedience lessons, and while the Collies will keep working at a slow steady pace, Honey gets bored with this, and likes to jump around from exercise to exercise - after 2-3 retrieves she's ready for some heelwork, after a few minutes of heelwork, she's on to the next thing. While the Collies will keep working for food treats, I have to mix it up and use play as a reward far more. A good trainer will be able to accommodate Bailey's needs in a group lesson...
Also remember that he is still young, and probably hitting adolescence... if you are getting frustrated, it might be worth pausing the training classes and just strengthening the commands he does have....
One word of warning, though.... never recall unless you can guarantee he will obey, or you are only training him to ignore you...
Use a long-line while he is still learning the ropes... find the one thing he will move mountains for...
Above all, learn to enjoy your dog... if you are happier with him, he will want to be with you more, and will do more for you!
Remember, Cockers were bred to work - if they were untrainable, they'd be no use on the shooting fields!
In my limited experience, I think Cockers need to know what's in it for them - you do see dogs who will just follow their owners like sheep. Cockers are intelligent and won't just blindly follow... find what motivates him (praise, toys, games, food, sniffing... whatever works) and use it!