My oldest daughter did EXACTLY the same thing when she was that age! I took pictures to show her future boyfriends:). Oh, it was horrible and I really feel for you! One time she "painted" a smiley face and said "Look, Mommy, its HAPPY!" with such glee when I came into the room. All I had to say was "Mommy isn't happy."
Making sure to put her in footed pajamas, even sometimes with the zipper backwards once she knew how to undo it, helped a lot. Or if you can find the pajama tops and bottoms that snap together, these would be easier since it is so hot during the summer. It was winter, fortunately, when my Emily chose to "paint."
Good luck, and just know that he will grow out of it. Take pictures so that you can have some fun with this when he's older and get some of your own back:).