Hi M.~
This is a very good subject. I too believe it is a healthy phase that he will soon outgrow. I wouldn't even give it the attention of a repremand. I will suggest this though as it has worked well for me..
Instead of saying no to him as much try giving him a choice between different directions to go in. For example: If he's drawing on a wall, instead of reacting with a no, give him the choice of drawing on paper or in a coloring book. Now instead of him feeling bad about himself for doing something wrong he can feel good about making a positive decision. And praise him once he has made a positive decision. He's now gaining a little positive controll over his actions. By giving him choices he can be proud of himself for making a good decision.
This is something you will have to practie at for it to be an instinct just like saying no is your instinct right now but believe me it is a positive way that will help improve his life in many ways.
And as the old saying goes...this too shall pass, lol. :o)
HTH,
C.