I did once see a set of rulers with wiggly shapes along them - the sort of shapes you may want to trace to make a fancy edge on a greeting card. If you don't work out what exactly he wants it might be worth a look in the card-making section of, say, hobbycraft.
I found the 'Wicked Uncle' website excellent for interesting age appropriate suggestions. They had things I'd never thought of. Great fun too. They deliver quickly and efficiently.
My grandson will be 7 next week. One of the things he has asked for is “something arty with different shaped rulers!” Does anyone know what this is, and where I can get it please?