I drew a circle that was small enough to fit on the pumpkin and asked him to draw a face on it. He wasn't happy with the first one, so he gave it another shot. When he was done, I taped it to his pumpkin and used it like the templates that come in the books you can buy (poke holes though the paper to outline the design, then carved the pieces out).
My son is thrilled, and so am I. I think the pumpkin came out pretty close to the way he drew it. It is now on our front steps, right between my pumpkin, and my husbands. I think it fits in perfectly.