He's 3? Let him choose. Isolate an area where you want the "sign" the project. (Draw a box in light pencil or something to provide boudaries.) Then explain that lots of woodworkers make a mark or write their names on their work to show everyone they made it. (You can even do a Mouseman Thompson number on it!) Ask your lad what mark HE would like to make. He may draw a pic of him and Dad with crayons, or just a circle. Whatever, it would be HIS mark and when he looks at the project 30 years from now . . . Whatever he does you can seal with polyurethane to preserve it.
FoggyTown