Icy:
If we know that something is dangerous, we will certainly say NO. And you know, my girl, if we say NO with firm voice, she will cry so what we do, we will say NO, NO, NO with singing tone. ;-) Funny right, but it works and my girl actually gets the message and gives up with a smile. ;-)
The one thing I see it's important is everybody in the house must agree on something when comes to "discipline" kids.
For example, when my girl tries to "test" our limit by allowing herself into the kitchen, we will ask her: Who is in the kitchen? Then imme she will step out of her forbidden zone and will not make any noise. We always don't allow her to be in the kitchen nor toilet except changing her nappy or bathe time.
When I say no to her when she is crying, my hubby will not give her comfort or hug- he will tell my girl: mummy says no, you have to listen to mummy or vice versa.
So far, she gets what we mean very well.
The next important thing is try to explain alot and alot to your boy. You will be amazed how he can understand you. ;-)