I think at this early stage of toddler-hood they also have little to no impulse control, so while he might understand what 'no' means, he just can't help but touch/do what he isn't supposed to. DD is usually pretty good, but when it comes to my laptop which sits on a table next to the lounge, she can't help but touch the keyboard/screen. she knows not to because she'll look at me and go "uh uh" (which is what we say to her along with no) then turn and touch it. The only thing that works is removing her from it and distracting her with something else. In time he will understand and obey, I think (IMO) he's still a bit young for actual 'discipline' like time outs, although popping him in his cot for a few mins might work. I did it last night with DD because she was throwing a tantrum at 1am (yeah..I have nooo idea what it was over, probably because I was trying to stop her going sideways on the bed so she didn't fall off the edge!) and wouldn't stop. Put her in her cot to scream it out for a few mins, until she would accept me picking her up to calm her down.