How to deal with bratty little kids?

Okay, I've known my best friend's little boy since he was born, and ever since I've loved him and coddled him as if he was my nephew. Well, I went away for a few months, and I come back, and he is such a brat! He turned three in the summer, and I'm coming to realize that he is a little hellion and I can't stand him. See, I tell him to not do something, in a nice way, and he'll keep doing right on purpose. I see the little evil glimmer in his eye, that he knows he's doing something wrong, and he takes pleasure in it. Plus he is very bossy, always meddling in what I do and telling me I shouldn't do this or that. So what should I do? Plus it also scares me because if I ever have kids, I hope they are not like him!