Try different types of hair, he could have a scruffy hairstyle which would give that cute, rough and tumble little boy look. Especially with the way you've made his eyes. Costuming will do a lot to cute him up.

You have the large pupils which helps make it look younger (general rule in the puppet world).  Agree by the others small nose, some hair and arms for big hugs would help him be "cute."  find some cute kids clothing at your second hand shop and you will have your boy.

Show us the end result, please.

Daryl H

The puppet is great.. add just some dishevelled hair made of a fluffy light material
Don't put to many things and leave him sort of "incomplete" and let the actor do the rest