If you get rid of him, at least give him a 'learning' explanation.
I started out washing dishes in high school and quit when I was 18 after 3-4 years. I think I finished up making .75 more than when I started.
Also have worked in stores like Banana Republic and ended up never getting a raise.
Jobs starting out at less than 10/hr rarely give raises and when they do it is usually like .50 and the company even makes a big deal out of that.
You did not give him realistic expectations (for future jobs like this) giving him a substantial raise every month