he was trying to be like his mommy. My step daughter did something similar when she was about 3 or 4. still not sure how she got the scissors in the first place.... anyway, she decided to do her clothes, and her sheets. And now that I am thinking of it, when I was living at home my baby brother did the same thing to his sheets. I know where they got the idea from, just never thoguht that they would pick up on what I was doing. Alot of my first fabrics were old sheets and clothes to see if i could make them "better". So they followed suit.
I learned to keep extra fabric- a pre-cut up pillowcase or 1/4 yard or "cool" fabric for the kids to cut up, and explain to them that that was the ONLY fabric they got to cut. My step daughter ended up making a train placemat, that we glued together. I never sealed it, so she couldnt use it at the table, but she loved to look at it in her room. and take the whole thing around and play "choo choo" with it :)