Age 5....Behavior

Now my little baby has pass to a little boy! Little Emilio have now 5 years of age and he is learning knew things like how to count, how to write, how to read and many other interesting things that he learned in school. This have been very helpful in the learning in little Emilio, in the school the teacher have tell me that he is a smart kid and that he have no problems in learning. He is learning to control itself and have a self concept of him. That have been a good start, since I'm a single parent and he is a single child. There are some things that I need to set, for example my authority over my kid. Normally little Emilio is well behaved but sometimes he touches something and I say no, he starts complaining about the situation. I just say No and the what it is! but I do it in a way to make him comprehend.