They make a special cleaner for flat screen monitors and TVs. You can buy it just about anywhere. Ours came with a special cloth too.
On the issue of the crayons, trust me...put them up. Have a designated area where he's allowed to color. It's not stifling creativity, it's teaching him boundaries. It's not appropriate to color on anything other than paper. No matter how well behaved your son is, it's just too tempting to color on the table, the walls, the monitor, the floor, the carpet, every door in the house, etc. I'm speaking from experience here. I made the mistake of leaving my girls alone with crayons while I went to the bathroom. In a matter of minutes they (ages 3 & 15 mos) had decided that the table looked prettier purple, the white trim would match better if it were blue, and my bathroom door was a great place to draw smiley faces. Luckily, it was all washable, but now the crayons are put up until we have art time.