What is a good Techniche?

I am babysitting tomorrow and every time I babysit this little boy he yells at me for something I told him to do! Like if he asks if he can have cake, (he has G.E.R.D, which means he can't eat dairy or chocolate or saucy items) I will say no and then he will yell at me " WHY NOT?!!!! " Then I pretend I call his mom but I don't...what is a good techniche that will make him stop yelling? (Already tried putting him in the corner)