What do you do when someone asks for more money, but you know they are already overpaid?

I hired a young guy with no experience to work with me and I offered to pay him what the big companies are paying their more seasoned workers. At the moment I am also paying him cash so I am essentially paying him out of my pocket and he isn't claiming it. He asked me the other day if I could pay him more so I explained that what he is making for his position and the hours he works is insane, and the only reason I paid him what I am is because I really like him and I know he can do really well with me in this industry. I am finding it hard to really drive this point home to him and want him to stay with me and be happy.

I don't want to lose this kid as he has made my life a lot easier and together we are generating a lot more income for the company. I told him that any jobs he finds, sells and completes on his own, 40% is his, and any job that I book for him and he completes he gets 10%. That would be on top of his weekly salary also. I think I can adjust these numbers a little as a start to keep him motivated but I am wondering if anyone has any other ideas?